I have a list... a big list to check every Monday, Wednesday, and Friday:
Collar Six - (NSFW, EXTREMELY NSFW AT TIMES) a tale of BDSM.
Sore Thumbs - A comic about the perils of a video game store.
Pictures of You - a slice of life kinda comic that goes into the lives of college students, it can get dark at times and NSFW at times.
Alien Dice - honestly at times it feels a bit like a spacey "Twilight" with how angsty the male lead is, but it's good. Too much to be able to really explain it.
Bittersweet Candy Bowl - fuck this comic, I can't stop reading this comic. It's cute but as the name suggests it is not the happiest at times.
Questionable Content - I love this comic. It cheers me up so much at times. Slice of life comic in a slightly more advanced future. IT's been mentioned on this thread already, I think.
Menage a 3 - Slice of life with an emphasis on relationships. Can get REALLY NSFW, but it's helarious.
Stick Dilly Buns - Kinda ties in with Menage a 3, Funny as all get out.
Sandra On the Rocks - Ties in with the two above.
Candi - College Slice of life with a super-powered ferret?
Misfile - Hilarious mix-ups when a boy suddenly turns into a girl, and a girl looses the last two years of her current life.
Perspacity - I can't even explain this one, really. Seriously.
Bear Nuts - IT'S LIKE CARE BEARS, FOR ADULTS.
Red String - it's a completed comic, but I feel it needs to be mentioned because of how good I think it is.
Devil Bear - can be somewhat adultish at times, a teddybear rules hell with the help of seven divas.
Flipside - READ THIS. FUCKING READ THIS. I just can't explain it but it's good. Can be adult, but it's interesting.
AND OF COURSE...
Homestuck - Four kids pretty much doom their planet to destruction and then set off on a quest. There are doomed timelines, mixed up timelines, shit you can't even begin to keep things straight with. And Pauses. Andrew Hussie likes to take time. It is soon to end, and he has stated this isn't even meant to be what he is known for. He has other shit planned.