As a driver: 95 in a borrowed Miata.
As a passenger: 105 in my friend’s '70 Chevelle.
Worst car to speed in: 85 in my 74 SuperBeetle.
150 in a Saab 900 (estimate based on tach reading–speedo only went to 85 that year by law).
More impressive IMO:
120 in a 1939 Caddy model #6119 (precurser of the Fleetwood)
No, it was level ground. Whenever I try to max out a car, it’s got to be level ground or it doesn’t count. 
I’ve never had a “hot” car, so my top speed as a driver is around 90, in a Celica. But there’s a funny story coming, so don’t laugh at my less-than-fast history.
As an “innocent” passenger, I’ve topped 120-125 or so. Pal of mine back in high school had a heavily customized Charger, and he used to take it out to a Nowheresville straightaway and open it up. At that speed, the engine’s a roar, and I felt like the car was about an inch and a half off the ground. Hell of a ride.
But as a “participating” passenger, my wife katrina and I had a rental Taurus on one of my business trips back east. We were driving from Boston to Vermont, late one night. We’re in New Hampshire, probably in the neighborhood of 10pm, holding a steady 80mph. The car seems to be fine at that speed (unlike my own Taurus, which wouldn’t have been happy at 80 even if it weren’t sitting dead in my back yard), and there’s nobody else around, so we decide to push the speedometer a bit, just for grins.
We top 90 – no problem. We figure we ought to go for 100, as long as we’re up here. She’s driving, so she eases down on the gas, pushing the needle higher.
At about the time we cross into triple digits, we notice a dark car floating up alongside us in the left lane. I say dark because we can’t make out anything about it, other than it’s not a truck. It’s just this black shape, creeping up on us. Even the headlights are dim. And remember, it’s really, really dark, and the New Hampshire rural freeways have no streetlights of any kind.
Anyway, it’s a little strange, and gets stranger when the shadow car stops accelerating and glues itself to our left rear quarterpanel. At this point, we have no idea who they are. It’s just a mysterious car that has drifted up behind us and is now staying right with us. Just another commuter? Then go ahead and pass. Weird psycho stalker type?
So after a couple of minutes of this guy sitting on us, we’re starting to get worried – and that’s when the blue and red flashers come on.
Immediate adrenaline rush, especially as we’re keyed up already by the thought of this guy being a wack job. We had ruled out the cop option; we’re hovering between 100 and 105, and we assumed we would have been pulled over immediately, instead of being watched for a while. In retrospect, it makes sense that a cop would eyeball a 100-mph driver for a minute to see if he could tell if maybe we were running from a burglary or something, or just a garden-variety asshole, before he flipped on the lights and announced his presence.
Anyway, we completely freak out, and instantly drop to about 30, assuming we’re about to get into serious trouble. (Nobody has mentioned rapid deceleration from these high speeds yet. A real stomach-turner, let me tell you.) But before we can go from 30 to stopped and contrite, we see the cop has turned off his lights and is now just continuing on his way, as if nothing had happened. Again, in retrospect, we figure the cop was probably on his way back to the station at the end of his shift, and didn’t need the hassle. Or, given the “Live Free Or Die” slogan on NH plates, maybe a 100-mph driver isn’t worth more than the metaphorical tap on the shoulder, and a casual, “Hey, slow down, eh?”
Anyway, we drove at 60 for about a half hour after that, which irritated the dozen or so other late-night drivers whipping past us, but we had to let our adrenaline-quivering muscles calm down. And we continued on to Vermont at a safe, subdued 80.
Neither of us has ever taken a car up to that speed since.
102 mph in a '69 MGB-GT in Olympia, Washington.
And you know what the “GLH” stands for, don’t you?
I spent quite a few fast evenings in my dad’s 5.0 Mustang rag top. Manual. He had the governor removed (the same as the State Highway Patrol Mustangs), and had some other stuff done to it as well. I’m not a car person, so someone tell me why he had the catalytic convertor removed???
Anyway, I’d take it with a buddy of mine on some Louisiana straightaway back roads and open it up. Since, as Anthracite mentioned, the top end with the governor was about 140 or so, I estimated about 157. Don’t know for sure. I do know it had the most amazing pick-up. Also, it wouldn’t start to red-line in 3rd gear until you hit about 90.
I wish I still had that car around.
I’ve gone 110 in my 92 Honda Civic. It was about 5 years ago, on a local road that’s very straight, and has little traffic. It scared the living hell out of me, and the fastest I’ve gone since is about 90 on the highway, but only for a short time. Usually I cruise right around 72.
Jman
My friend Bill Hamilton had a Mustang Mach 1. His Dad had brought it from a buddy who did a lot of drag racing, and the thing had a 351 windsor, 4bbl, and god knows what else done to it. Thing had about 700hp.
First time he took me out in it, his joke was we were doing the speed limit (35mph) without hitting the gas. Idled up a little, I’d say.
First time he actually hit the switch (so to speak) was to pass some mentally-challenged person on a ninja. Damn. It was like a rocket sled. Went from 45 to about 90 on a back street here in Jackson in nothing flat, it seemed. Guy on the Ninja gave up at 110. Too chicken to take the curve on a bike, I think. I know for a fact the Mach 1 wouldn’t have had a problem.
He did that a lot. Just used the engine to pass people. You know, fluff cars like Mustangs, Corvettes, Porsches. 
Top End: We have no clue. Best we ever did was nail the speedometer at 140 passing about 7 cars in a slow caravan blocking two lanes of traffic. We were in the oncoming lane. I have no doubt that thing could have done better, but it was set up for accelleration.
Personal Best: 102, 1970 Pontiac Boneville 400, 4bbl, turbo 400. Was just floating along, peacefully. Wonderful feeling, still connected to the road and everythng.
Sigh.
90 in my SUV on route 50.
Family cars aren’t fast, they’re big.
“What’s the fastest you’ve ever been in a car?”
—Well, one time, in the back seat of my '67 Chevy, Paul got to third . . . Oh, wait, did you mean . . . ?
. . . Never mind . . .
I went over 80 the other day in my minivan. I thought I was gonna see Jesus, man.
Because the catalyst creates an exhaust restriction that saps power from the engine. This has a pronounced effect at conditions of high load and high engine speed, such as top-speed testing. Reputedly, on my 1998 Mustang GT having an open-flow exhaust will increase the top speed from 144 to 150 - but I stress “reputedly”.
However, not only is this not legal to do for most cars built after January 1, 1980, but it also is not so good for old Mr. Environment if everyone does it.
Let’s see…
I went around 110 in my friend’s Camaro.
Around 90 with the top down in another friend’s Cutlass.
I think it was about 120 downhill in Masachusetts in my friend’s dad’s Crown Victoria (they use those things for cop cars. We also tested the pickup on it. 0-60 in 6 seconds.)
Car’s are fun.
127 mph.
When I was younger (and stupider) on the 605 in LA at 3AM one night I achieved that personal record. What made it exceptional was that it was in a Honda CRX, the little two seater from the 80’s that weighs 1800 pounds and has a 1.5 liter engine. It took me a full minute to go from 105 mph to 127, for some odd reason 5th gear doesn’t pick up as well as the others 
I estimate it was 127 because the spedometer only reaches 120. The needle hit the pin well below that.
Miscallaneous fact from personal (and anecdotal) experience: don’t touch your brakes if you’re going over 100mph. Let the engine slow the car down to a manageable speed before applying controlled braking. Tapping the brakes (even lightly) at a high enough speed will send you out of control.
Polarfield
Nothing personal, but either your car had shitty brakes or you’re not a very experienced driver, or both. My advice for high speed runs is DO NOT make any sudden movement of the wheel. That will put you in the twigs. You might get your brakes hot, but if need be, use 'em. Just don’t lock them up, but thats another thread 
126 in my '87 Buick Grand National on I 95 South near Lumberton SC. I wanted to go faster but the cars computer shuts down fuel injectors at that speed.
i’ve gone over 100 mph on my mom’s Suburban. it was on the way to San Fran from LA. i probably went faster because after you reach 100, the turbo kicks in and the speedometer only goes upto 100, but i could feel the truck going faster and faster. I felt the Suburban get lighter, so i slowed down, but i made it to San Fran in under 5 hours (it usually takes 8 going 70mph).
I’ve gone 150 mph on my old car, a 1996 Nissan 300zx. I loved that car, but i had to return it. 
Alchasehy
140 mph, several times in my friend’s Dodge Dart Swinger (yes, an econo-box with a 340 with 2 4-bbls could do that - timed by another friend’s brother who had an (I forget - was it) L-88 Corvette (the aluminum(?) 454 - the 150 mph 'vette). Once in the trunk, as I was being smuggled in to the drag strip.
My current personal car puzzles me. I’ve only taken it to ~ 90 mph on country roads, but it’s speedo pegs at 160 mph. It’s a BMW 735i, kept current. Is this car really capable of anything like what the speedometer purports to be able to report?
120 mph 63 bonneville
Then almost constant 100mph from Estes Park Colo to Ia City Ia. I80 was under construction then.
My brother and I got up to 110 mph in his 1992 Toyota Celica GT, passing somebody outside of Columbus, Ohio. This was back in 1993.
The above desn’t sound like much, but I can top it. I was riding with my sister in her 1979(?) Ford Fiesta- and she got it up to 85! I seriously thought I was going to die. At that speed that car would’ve disintegrated if we’d hit a squirrel. It was a major piece of shit.
For those who don’t know anything about Ford Fiestas…they’re like a Geo Metro, only more tinny. My sisters’ had a hole in the floor of the passengers’ side. I think the whole car weighed about 1000 lbs.