A couple of milfs to start us out:
The pathological liar Glade scents lady.
The AT&T Rollover minutes mom.
The ever so slightly incestuous Nicole and Papa, a series for the Renault Clio in the UK.
Don’t know if they made it to the US, but here’s a good example, with the added bonus, for Brits, of including the Ski Sunday theme - pure gold.
I had a big crush on Nicole.
Quite a while back now, a young yuppie had just gotten her first American Express card, and wanted to take her boyfriend out to dinner to “break it in”. She was hot.
Joe Isuzu. (Darn, can’t find good ones on YouTube)
The Fed Ex guy who talked really fast.
Oh, how I loved the eighties.
There’s another pathological liar running about these days, claiming that her secret to her delicious chicken soup is something along the lines of “a high degree of culinary intuition coupled with years of experience.”
By which she means (as her guests deduce) Swanson Broth.
God, I hate those commercials. I am so NOT inclined to buy something based on the advice of someone that is clearly not telling the truth. What makes anyone think I would?
Personally, a more positive contribution: Hayden Panettiere (the cheerleader from “Heroes”) in a commercial for Hershey’s Cookies and Creme candy bars. She played a girl scout going door to door selling power tools (since all of the cookies had gone into the candy bars). This was years before she was on “Heroes”, but she was excellent.
I also remember from my youth the “Hi, guy!” guy from the old Right Guard commercials. Those used to creep me out as a kid. How would YOU like to look in the mirror, and not only see someone else who talks to you, but reaches through the mirror to snatch your pitstick?
That would freak me right the hell out.
The, “Yes, I am,” guy from the Budweiser(I think) ads in the mid 90’s
“You mean, Dr. Galacawicz?”
“Yes, I am.”
Mr. Whipple “Don’t Squeeze the Charmin!”
The Tidy Bowl Man
Charlie the Tuna
The “Can you hear me now” Guy from Verizon
The FreeCreditReport.com Guy
The AFLAC Duck
Hmmm…maybe I do watch too much TV…
Raymond J. Johnson, Jr.
You didn’t say they had to be fond memories, right?
I miss the cranky Scotsman that was in the ads for Keith’s India Pale Ale a few years ago. He had some great memorable lines, like “Stand and be in awe of Mr. Spilly Pants!”
Too bad it turned out that the actor wasn’t just fond of rather crappy beer from Nova Scotia. He was into the kiddie porn too. 
Well, being kinda old, I always remember
Choo Choo Charley from the old Good n Plenties commercials (and even as a kid, I hated Good n Plenties. Blegh!)
The Frito Bandito
The Taco Bell Dog
Mrs. Olsen.(how many neighbors did she have anyway?)
The Big Fig Newton
The Ancient Chinese SecretGuy
The Chuck Wagon(as a kid I liked this one a lot)
Ralph, the guy who couldn’t believe he ate the whole thing (from the Alka-Seltzer ads in the mid-70s, I think).
Currently, I like the lady who doesn’t believe in dentists in the elevator with the 1-800-Dentists guy.
In the late 80’s - early 90’s, Boatmen’s Bank (a long-gone regional bank) had a series of commercials featuring the Boatmen’s Guy, a balding, dry-witted guy who would pop up out of nowhere and start talking to people about the bank. My favorite one started with a guy on a riding mower, mowing his lawn. After a couple of seconds, the Boatmen’s Guy, in a trademark dark suit, zooms up on a riding tractor of his own and starts spouting off CD rates or something. He zips off at the end of the spot, calling, “You can run but you can’t hide from the Boatmen’s Guy!”
Yeah, I remember Ralph. I still use his catch phrase when i over eat!
Poetic Justice. I fell asleep soon after it started. I was in the theater with a friend, his girlfriend and a blind date they were trying to set me up with. I couldn’t take it, so I offended all of them when I announced my intention to go find better entertainment. Like watching paint dry.
The cavemen from Geico.
The young black guy in the WaMu “stuffy bankers” ad.
Billy Mays.
That’s probably Thom Sharp, who also played Tim Taylor’s brother Jeff on Home Improvement. Southwest Bank in St. Louis has apparently just hired him to do some commercials.
The old man in one of the UK’s most famous ads, searching for a certain book.
Eagleman, the (apparently intentionally) poorly-done Chicago insurance mascot.
Mr. Owl, who has a dubious answer to the immortal question of the Tootsie Pop conundrum. Voiced by Paul Winchell, the ad is one of the longest-running in history. Here’s the rarely-seen full version, featuring Frank Nelson and Paul Frees.
She was the very first I thought of upon reading the thread title. You know her husband often wonders why he stays with her, then sees her naked in the tub and says, “Oh, right…the blowjobs.”
I’ve only seen that one once or twice, but my impression was that she was kidding and they knew it.
Joe Isuzu was great. I’m surprised there aren’t more of those commercials available. Some of them on YouTube are OK, but not as good as the first one with the character.
The Federated commercials were a hoot. I was hoping they’d have the one where he’s a viking and he says, ‘I’m Fred R. Rated, and this is my wife Unda.’
And then there’s Crazy Gideon’s commercials.