I can do Goofy’s “Hyuk!” laugh.
I can also do Will Ferrell’s Harry Caray. “If you were a hot dog, and you were starving, would ya’ eat yourself?”
If I concentrate, I can do a singing Load/ReLoad era James Hetfield. Same for a post-Alice in Chains Jerry Cantrell.
Another Marvin the Martian here - I make people blow milk out of their noses when I do that one. “The Eludium Q-38 Explosive Space Modulatoorrr! That creature has stolen my SPACE MODULATORRRR!!!”
Beyond Marvin, I do a pretty fair job with Jimmy Stewart, Henry Kissinger (especially when he did the weather on CBS that once…) Ronald Reagan, Jimmy Carter, George Bush Sr. and a conglomerate of Kennedys (Jack, Bobby, Ted - they all pretty much sound the same)
I can do a selection of other movie stars in character - Brad Pitt from Fight Club, Jack Nicholson from The Shining and One Flew Over the Cuckoo’s Nest - wait, Jack Nicholson in pretty much anything sounds pretty much the same, doesn’t he? I do Kate Hepburn in On Golden Pond “You Old POOP!”
Mostly, though, I do a much better job at generic dialects. I can easily pass for a Brit among the English, a Southerner in Alabama, a New Yorker in the Heart of Brooklyn and a Bostonian in downtown Charlestown (OK, that last one’s a bit easier - I did grow up south of Boston…)
“I’m going to DIS-integrate you!” <heavy panting and furrowed brow>
My normal singing voice (which has been described as a Cajun Elvis) isn’t far off from Louis Armstrong so that one’s pretty easy for me. Sometimes I even do him unintentinally, I have a wav of me singing “Jambalaya” in which I drifted into Satchmo at the end.
I can do Fat Albert, too. I just take Louis Armstrong down about an octave.
I do John Wayne about as good as anyone on “Whose Line”.
Given my nomal nasally speaking voice, Sly Stallone as Rocky isn’t much of a stretch.
I can sing the Alphabet Song (a b c d, e f g…) as Bob Dylan.
Well, let’s see. First I’ll start with actual impressions:
Homer Simpson - in certain situations
Grampa Simpson
Apu - I have a friend who always want me to do Apu for him
“Ah, the seering kiss of hot lead, how I have missed you. I mean, Help, I think I am dying.”
Marvin Martian
The Sea Captain (from Simpsons)
Then there are my generic accents:
Scottish
about 3 different english accents (cockney, high british, etc… anyone who’s been to England can tell you that they have LOTs of different accents there)
Russian
“old jewish guy”
Most of the distinctive “american” accents (southern, New England, Minnesota, Cali Surfer dude, New York Italian(i.e. “mobster”))
I’m sure there’s more, but those are the ones that come to mind immediately.
I have no doubt that you do a great JEEEEOOOORRRRRBB!
I also do a clucking chicken, a meowing cat, a Scottish accent (if I watch Trainspotting first to get in the mood), a couple of British accents, and Forrest Gump.
I’ve been told that my Neil Diamond impersonation is pretty good. I also do a decent Tom Waits, esp. after a night of drinking scotch, smoking, shouting at the moon, reading Bukowski, hanging out with hookers, etc.
I do a dead-on Leon Redbone. This skill is very rarely appreciated.
Cartoon characters:
Marvin the Martian
Bugs Bunny (fair, not as authentic as I would like to)
Sylvester
Rocky and Mugsy
Beavis and Butthead
Rocky and Bullwinkle
Barney Rubble
Snagglepuss
Yogi Bear
Boo Boo
Tennesee Tuxedo
Hank from King of the Hill
Real people (or their characters):
Ross Perot
The Church Lady
Howard Cosell
Ted Koppel
Tom Brokaw (somewhat)
Robert Stack
Ronald Reagan
As a side note: I used to have a co-worker whose voice I could imitate perfectly, enough to fool other workers into thinking it was him whenever I used his voice (when they couldn’t also see me, of course). It was good for him that he was on my good side. If he had been an employee I didn’t like I could have gotten him into all sorts of trouble. 
Some more of the list:
Marvin the Martian
Fat Albert
John Wayne
Kate Hepburn
Bob Dylan
John Cameron Swazye
Jerry Lewis
Popeye
Horton the Elephant
Fozzy Bear
Beaky Buzzard
Yogi Bear
Boo boo
Snagglepuss
Barney Rubble
Fred Flintstone
Alfred Hitchcock’s “Goot Evening”
Bela Lugosi
Peter Lorre
Adolph Hitler
Desi Arnaz
Lucille Ball
Sergeant Schultz
Werner Klemper
Walter Brennan
Eric Idle
The Cowardly Lion
Animals:
Perfect wolf howl
Dog bark
Sheep bleat
Lamb bleat
Goat bleat
Kid bleat
Duck quack
Chicken cluck
Chicken squawk
Turkey gobble
Pigeon cooing
Crow cawing
Cow mooing
Puppy yapping
Cat yowling
Panting dog
Pig grunting
Pig squealing
Frog croaking
Wildcat
Cat spitting
Cat hissing
Cat growling
Crickets
Mosquito
Hummingbird
Kookaburra
Chimpanzee
Monkey
Objects:
Creaking hinge
Klaxon horn
Most voices of the standard car alarm
Uncorking a bottle
Kermit the frog
Lilo (or is it stitch? I’ve never seen the movie) from same movie
Howard Cosell
passable Jimmy Stewart
some random Jim Carray bits
generic Beatles voice
Apu (very well)
Mush-mouth fom Fat Albert (even better than Bill Cosby did)
The Jackalope (perfect imitation)
Steve Irwin the croc hunter (almost made a girl laugh hard enough to pee herself with that one)
Various cat sounds
Dental drill
An ok Jimmy Stuart and Louis Armstrong
MonkeyMensch you can do James Earl Jones? nice, he’s got an awesome voice.
Me? Practically nothing.
Oh, god, I love doing voices! Nothing would make me happier than to quit my day job and become a voice actor for cartoons. As far as voices I can do, I spent way too much time doing Beavis & Butt-head back in their heyday, so I’m pretty good at that. Lately it’s been any number of Homestar Runner characters - I know “anybody can do Strong Bad,” but that doesn’t make it any less fun. I think my Tennessee Tuxedo and Chumley are pretty much dead-on. Being the fan of stupid humor that I am, I can do a number of Jerky Boys characters (Saul Rosenberg and Tarbash being my favorites).
Sometimes I like doing animal calls, too - I used to freak out my cat by making the “kitten-in-distress” call when she had kittens. She’d come into the room and give me this look like, “WHERE’S MY BABY?!? WHAT HAVE YOU DONE?!?” I can do a passable croc/gator baby distress call, too, but that’s likely to get me killed if I’m not careful. Still, I doubt I could resist trying it on an adult. I’ve tried to get the hang of some bird calls, but as they’re mostly whistles they’re trickier, often exceeding my whistling range at the high or low end.
This is my impression, too. I get compliments on it.
Michael Myers doing the Scottish accent… “Oh, now he’s gonna cry 'imself ta sleep on his huge pilla.”
And I can imitate pretty much any accent. My Texan friend hates it when I imitate hers; I’m not making fun, I just subconciously practice the accent. My dear German friend doesn’t mind when I exaggerate hers.
“My Gott I chust vant to buy some cheans!”
Just remembered. There’s a .wav file that some people use for “error” on their computers that sounds like a very high-pitched “Uh-oh!” I can imitate it perfectly.
And I do a dead-on Luciano Pavarotti, specifically when he sings Christmas songs in English on The Three Tenors Christmas album.
Homer Simpson
Mr. Burns
Barney Gumble
Bill Clinton
Dr. Phil
Richard Nixon
Shaggy
Scooby
Barney Rubble
Peter Lorre
Alfred Hitchcock
Yogi Bear
Squidward
Elmer Fudd
Beavis
Butthead (much harder than Beavis to get right)
Bullwinkle
Olive Oyl
Popeye
Mickey Mouse
Jan Brady
Singing:
Johnny Cash
Elvis (both young and old0
Gordon Gano (Violent Femmes)
Lou Reed
Louis Armstrong (until my throat shreds)
Oh man, I love doing voices. It’s something i’m always doing, in one way or another. Among them are the following:
Marvin the Martian
Bobcat Goldwaith (this one flat out slays everyone when I do it.)
Wakko Warner
Yogi Bear
Barney the dino (usually done in some sick perverted way)
Mickey Mouse (same as above)
Jimmy Stewart
Sean Connery (still working out the kinks on this one)
Ross Perot
George H Bush
John Wayne
(the above three I do in succession, one leading into another…learned that trick from watching Robin Williams one night on the Tonight Show)
Lt. Worf (got the voice and inflection down, but cannot get volume on it for the life of me)
Elmer Fudd
Jackelope
Kermit
Apu (or generic middle eastern “joke accent”)
Beavis
Bob and Doug MacKenzie
Barney (from The Simpsons)
Popeye
I can do a pretty good William S. Burroughs, and a fairly decent Louis Armstrong.
Both of them eventually hurt like hell if I keep ‘em up for any length of time, though – especially the Satchmo deal. For some reason, after a drop of the creature, it sometimes amuses me to sing hillbilly jug-band type songs in the Louis Armstrong voice-- especially the Holy Modal Rounders’ Glue’s in the Bottle.
Reciting Jabborwocky in the Burroughs voice is good for a giggle, too, or parts of Father William, 'natch:
I would appreciate it. Especially if you sing “Frosty the Snowman.”
I can say “propane and propane accessories” like Hank Hill.
I tend to mimic cadences more than vocal sounds. Female comediennes especially. I find myself slipping into Paula Poundstone talking about the cats more than I mean to.
And there’s a professor here that I do a fairly good imitation of. I was talking to another professor one day and slipped into the impression without realizing it while discussing the other professor. The professor I was talking to said, “Does he know you can do him?” I realized what I’d done and said, horrified, “Oh no! And please, please don’t tell him.”