There are a few good ones to look out for.
How I Met Your Mother - kind of like friends
Entourage - about a hollywood actor and his mates. Hilarious
Scrubs - has gone on for a while, and the upcoming series will be the last with the current cast, but was fun while it lasted.
Sopranos - Watched it from my teens. Was great.
The Big Bang Theory - Light hearted comedy. Is good, but nothing great.
Heroes - Gone of the boil after a an amazing 1st season. The new series has started of ok, so might be worth starting to watch it.
House - Intense drama, defo worth watching.
30 Rock - Is not that well heard of, don't know why, wins all the emmys. Fun comedy.
Gossip Girl - Ok, watch it mainly cause of my girlfriend. But is not bad actually. Good for teens.
Californication is good, but has a bit of adult content. So does Entourage I guess. Dirty Sexy Money had a good 1st season, havnt seen it since, and despite the name its not really an adult themed thing.
Family Guy is funny, though not really a drama or anything. Its spinoff, The Cleveland Show has just started too.
If you like adult comedy, then The Chappelle Show, although over is worth watching, if just for the Rick James sketch.
Also, if your a teen, watch The Hills, if you can bear it. All girls watch it, you will have something to talk about with them. Trust me, thats a great thing. And 90210.
Yeah I know, I watch a lot of TV. But its stuff I need to watch to get to sleep. Can't sleep otherwise.
Finally, in the UK Skins is pretty good. And The Inbetweeners is pretty funny. Both are on Channel 4/ E4
Yeah there are sitcoms and dramas, but good tv is good tv.