Community and The Life and Times of Tim.

I have only seen season 1 of Peep Show, and I also really liked That Mitchell and Webb Look. I may have liked it better, it is Highlarious! Numberwang and Sir Digby Chicken Caesar are awesome.

The Guild - I was shocked by how much I enjoyed it
Better Off Ted - Like the other poster said, how Modern Family has succeeded like it has while BoT was barely noticed amazes me.
Veronica Mars - Not really a comedy but worth mentioning because it was incredible.
The Increasingly Poor Decisions of Tood Margaret - On IFC right now. Cross and Arnett. Hilarious.
Important Things with Demetri Martin - I'd imagine he's got a pretty love/hate style, and the endless puns are fairly hit or miss, but it's an enjoyable show overall.
Ten Items or Less - Everyone I know hated it. I thought it was hilarious.
Parks & Rec - It gets compared to The Office (US) too much for my taste. It's not that good but it's still very funny.
Hotel Babylon - It's pretty cheesy, soap opera satire but I liked it.
Californication - I only watched the first season (and I've heard iffy things about the rest of the series) but little beats Duchonvy.
Also:
CTRL
The Inbetweeners
The Mighty Boosh

the league, FX after sunny, its a satirical view of fantasy football

The Inbetweeners (UK)
Although that's pretty popular and hysterically funny.
My Boys (specifically seasons 1-3).

Actually, I discovered Peep Show on BBC America a few months ago. The DVR was recording an episode or two a week, but stopped with series four. I've yet to watch the rest online, but it's one of my new favorite comedies. Other favorites have already been mentioned, Arrested Development, Always Sunny,Community, and How I Met Your Mother (which is surprisingly really good, despite the laugh track). The Venture Brothers on adult swim of the cartoon network is a hilarious underrated show, probably my favorite animated comedy.

Peep Show with David Mitchell? From England? That's pretty damn obscure for people in the states. That doesn't even show on BBC America here.
Bender: I was God once.
God: Yes, I saw. You were doing well until everyone died.

surprised nobodys mentioned fantastic "Peep Show" !

Slings & Arrows is the greatest show no one has ever seen or heard of. it will be your new favorite guaranteed. youre welcome.

I loved Veronica Mars. Totally different type of show, but really good, nonetheless.

Most have already been said but here it goes anyway:
Reaper
Better Off Ted
Dead Like Me
Undeclared
Freaks and Geeks
Psych
Futurama
Bender: I was God once.
God: Yes, I saw. You were doing well until everyone died.

It's Like You Know

Bored to death bores me to death. Too bad hbo didn't pick up Party Down over that show. I don't think we'd be lamenting it's cancellation if they had.

bored to death on hbo is hilarious i have never been a huge ted danson fan, but he steals the show.
while i wouldn't classify it as a comedy, i think fans of party down would enjoy veronica mars. it definitely has funny moments and witty dialogue. the creator of party down also created veronica mars, and many actors from veronica mars have appeared in party down as main cast members or guests.
check out veronica mars - seriously, it's an amazing show!

Eastbound and Down
"Can I wear the mask from Scream while I do you from behind?" Kenny Powers

Man, I'm really enjoying Party Down. Just found out about it a few days ago. Watched the second season on Starz On Demand and almost finished with the first season via Netflix (I don't want it to end). Some of these have already been mentioned but:
TV:
Curb Your Enthusiasm
Arrested Development
Spaced
The Mighty Boosh
The League
United States of Tara
Community
Futurama
Web Series:
Childrens' Hospital (Second season will air later this summer on Adult Swim Ken Marino (Ron) is part of the cast.)
The Guild
Legend of Neil

I hate sunny in Philadelphia. Characters are over the top and I hate them, nothing is relatable, most things they do are boring, plot is weak, everything is too unreal.

Oh I loved Knights of Prosperity! Sofia Vergara was so funny on that. I also loved the African-American guy who played Rockerfeller Butts. Pushing Daisies was adorable too and I was sad when it was cancelled. I also liked Undeclared, Extras, and I think I've seen every AD episode about five times. There is no other show out there that even approaches the genius.
The only quirky ones on tv now that I like are Weeds, Parks & Rec (except Rob Lowe who sucks in this), and the Office. Grim out there nowadays.

speaking of archer (which i love) if starz had put in half the promotional effort as archer maybe it would still be on the air.
ive been seeing ads for the new season of archer for weeks and it doesnt start until JANUARY! i never even heard of party down til the second season was over.
a little awareness can go a long way.

Archer, Louie are other good new comedies on FX. A couple of shows I found on netflix that are good are Pushing Daisies, Firefly and Lucky Louie.

Dog Bites Man was another good show that was its own type of comedy as well as Reno 911 and Knights of Prosperity.

"Action" was a great show that didn't last very long
A book hasn't caused me this much trouble since Where's Waldo went to that barber pole factory

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Titus - Brutal show, so funny and sad at the same time
Definitely. That show was brutal funny but sadly didn't get the attention it deserved.
I am not going to mention Arrested Development (damn, I just did) because I think it got afterwards much fame and attention - well deserved to say the least.
It's Always Sunny in Philadephia is actually quite successful and gets always renewed for another season. Of course successful means in relation to the channel's overall viewers.
Better Off Ted - That show was criminally overlooked. I don't get why Modern Family (great show) is so successful whereas Better Off Ted failed so miserably. BoT is atleast as good as Modern Family. Only weak part is the girl who is playing Linda. The actress is not bad but compared to the rest of the cast she appears as an outcast.
Have a moment of silence for my Xbox360 killed by the ruthless & evil RRoD