I would definitely recommend Jupiter and City of Blank.
Jupiter usually updates several pages a week, and is probably my favorite webcomic right now. It's Sci-fi. Very good writing and well done art.
I'm not sure how often City of Blank updates (the author is in the middle of moving right now, so updates are a little bit late), but it has a decent amount of world and story building so far, and I'm really enjoying it! Not sure about the genre, author has it listed as "action", it definitely has some fantasy/sci-fi elements to it.
In addition, I'll recommend Stop Looking at All Those Stars (which is kind of vaguely similar to gag-a-day, but with a sort of mind-screwy, sci-fi twist), and you know, my comic, Crow Summer, (which is a drama/urban fantasy about student vampire hunters and which is brand new, but updates once a week).