My top favorites:False Positive
A horror anthology comic with an engaging art style and some great twists. There's a lot of Cronenberg-esque body horror with no shortage of violence, but there's also a lot of Twilight Zone style twists and turns. There's also lots of dark humor throughout. It's not updated as frequently as I'd like, but the backlog of episodes is enough to keep you busy.
Another horror comic, this one by Chainsawsuit's Kris Straub. It's set in a distinctly Lovecraftian 1930s Midwest. Kris brings lots of skill to the table: a delightfully cartoonish art style, good comedic chops, and some good creepy horror. Unfortunately, updates have basically stopped, but the first two chapters are a great read.
This one's pretty popular, so you may have seen it before, but it's one that I regularly check. David Malki uses a lot of Victorian-era artwork to craft his comic, which is a fun style. This is a "clever" comic, not as heady as xkcd but with some social and political commentary that you don't get until the punchline. It's updated thrice a week, and the 13 year archive has plenty in store.