Bored To Death

Beggars and Choosers
Pushing Daisies
The Yard
imagine
Oz
set in a schoolyard; freakin' brilliant!
Titus
The Newsroom
the Canadian one, which was big here and got the BBC backing other projects by Ken Finkelman, but should have gotten more respect elsewhere. I still say
The Office
cribbed much of its approach from it.
Foreign Objects
the CBC/BBC co-pro Finkelman series. The Jesus episode is the funniest half-hour of television ever made.
The Fall and Rise of Reginald Perrin
Veronica Mars
Slings & Arrows
Pond Life
the British cartoon
The Job
Studio 60
Sports Night
(before they added that damned laugh track)
Twitch City
Misfits
(this side of the pond)
Harvey Birdman
And I wouldn't have put
Community
or
Archer
as 'overlooked' so much as 'underappreciated', but I see others are listing it.
All roads lead to truth if you're willing to travel honestly.

Since no one else mentioned it I'll have to: Grandma's House. http://www.imdb.com/board/11706532/ I'd say it's on par with Peep Show.

Workaholics. Definitely not super clever; but once you get used to the style/humor it's really good. Mitchell Hurwitz(Creator of Arrested Development) guest stars in a great episode.
Parks and Rec is also great.
NewsRadio is pretty amazing too. People mentioned that earlier. It wasn't the same show when Phil Hartman passed. That goes without saying though. I couldn't even get through Season 5. It was too painful, ironically enough, since I didn't know Phil personally. But Bill McNeal is one of the greatest fictional TV characters of all-time in my opinion. Definitely recommend this show if you haven't seen it.

Strangers with Candy.
The Red Green Show.
SCTV (probably not overlooked)
AD. Brilliant but not overlooked.
"Everyone's laughing and riding and cornholing except Buster."

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Wilfred AU
Wilfred US
Both great shows
fortunately i have seen everything else in this thread, epic.
Insert @V@T@R

I don't watch TV; I don't even own one but 'Community' is genius and is what led me to 'Party Down'. 'Arrested Development' is in the same vein but, personally, I never really connected with that show. I've also heard very good things about 'Cougar Town' but I haven't started that one yet.

"That's pretty dangerous; building a road in the middle of the street."

Withnail & I.
I know it's not a show and is definitely a bit darker than Party Down, but it has the same struggling actors vibe. Incredibly excellent.

I'm surprised no one mentioned before
Trailer Park Boys
Corner Gas
Both are Canadian comedies and in my alltime favorite list with Arrested Development, Curb and Sunny.

Not only was The Job funny, but it was meaningful. The episode where everyone descended into their personal addictions was seriously powerful. It was the first of the Leary trilogy, with Rescue Me and Sirens completing the first-responder trifecta.
My all-time favorite overlooked black comedy is Testees, about two ne'er do wells who subsist as guinea pigs for a horribly evil pharmacology testing firm. As a medical researcher who partially funded graduate school as a test subject (yes, I had nine, count them 9, bronchoscopies as a means of paying rent, who then went on to administer similarly evil tests (intradermal capsaicin anyone?), the creators and writers got this one down pat. It is slightly crazier than Black Books and Peep Show if that can even be imagined. I am cracking up at the moment just thinking back to the new erection drug.
Also, don't know if Documentary Now can be considered overlooked as it is still new, but it is a brilliant low-key mockumentary effort. Hader deserves an emmy for his many different portrayals as classic documentary subjects. And you can tell he and Armisen are also having a blast making the shows. The pilot where they play a pair of crazy spinsters was Monty Python level brilliant, as was "Juan likes Chicken and Rice" (the one which will win an emmy).
My Chimp DNA seems to have lost its password temporarily. Sluggr-2

Assy McGee, Psychoville.
Cheers to the poster who listed Dark Place.
Poets are made by fools like me, but only God can make STD.

One that no one else has mentioned -
The Job
, with Denis Leary. I defy you not to laugh multiple times per episode.
If you know your own mind, what is real and what is not, then do as you see fit.

It's Always Sunny in Philadelphia
The League
Louie
Blue Mountain State

In no particular order:
Childrens' Hospital
Community
Happy Endings (getting bigger numbers now)
Archer
Futurama
Family Guy (first few seasons)
American Dad (first few seasons)
Action (brilliant funny show)
MST3K
Raising Hope
The Powers That Be (NBC, early 90s)
I'm sure I forgot some.

I don't have time to read this whole thread, so sorry if someone mentioned it, but
Spaced
My god, do I love that show. Not only is it absolutely hilarious, it's also a pitch-perfect prefix for the career trajectories of Simon Pegg & Edgar Wright.
You can see that they had their style nailed from the very beginning.

The Comeback
was indeed a good show, but I couldn't see the premise lasting more than two years.
All roads lead to truth if you're willing to travel honestly.

Portlandia on IFC starting Fred Armisen & Carrie Brownstein

Can't believe no one has mentioned this, but: "The Comeback" which was canceled after ONE season. It was on HBO in 2005. Camera style mockumentary starring Lisa Kudrow as an out-of-work actress. It's BRILLIANT. If you're into awkward realty comedies like The Office, Party Down, Arrested Development, give "The Comeback" a few episodes and see if you're hooked. One of the best shows EVER in my opinion.

My name is Earl
Better off Ted
Dead Like Me

Ok, I agree with the others who said that Newsradio was brilliant. Other than that one, I don't have any TV funny shows that I think were overlooked. Oh, I just thought of some:
Absolutely Fabulous
Cybil (which I think was trying to be the American version of Ab Fab)
I really liked Veronica's Closet and Caroline in the City when they were on the air, but I from what I remembered those shows were pretty popular in their day, so I hesitate to categorize them "overlooked."
These movies are really obsure and really hilarious. They are must-sees:
And God Spoke (Newsradio's Andy Dick has a hilarious cameo as Abel.)
A Midwinter's Tale (British Title: In the Bleak Midwinter) Anyone who has ever been involved with theater at any level will love this movie. Also, Ab Fab's Julia Sawahala (sp?) is a would-be Ophelia.
Still Crazy (The seeds of Bill Nighy's performance in Love Actually are sown in this movie.)

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Community.
Who are you?!
I'm the Doctor!!

Some more British shows that haven't been mentioned:
15 Storeys High - very low key sitcom with sketch show elements. Takes a couple of episodes to really get into it but once you do it's laugh out loud funny. A lot of full episodes available on Youtube
Garth Marenghi's Dark Place - a spoof 1980s hospital drama/horror. The worst 'acting' you'll ever see on TV, hilarious!