One of the online comics (over on the sidebar on the right side of blog, there) that I particularly enjoy is Derelict. I'm not sure how much I can say about it without giving away spoilers, and I really don't want to do that, because one of the things that the comic does extremely well is not over-explain its world. The characters who live there take most of the strangeness for granted; they're used to it, after all. That said... well, it's a world that, if not exactly post-apocalyptic, has undergone some very radical (and vaguely Lovecraftian) changes from the world we know. The writing is good, the art is excellent, the setting is fascinating, and the whole thing is well worth a read.
However, if you're not a big fan of reading comics online (and personally, there are plenty of times when I prefer to do my reading from actual paper books) there is now an alternative: Derelict, Book One: Deluge is now available as a graphic novel. If you'd prefer to read a complete story arc all at once, rather than following along on a page-by-page basis online, well... now you can. (I'm still waiting on my copy of the graphic novel, but I really enjoyed following along online and have no reservations about recommending it sight-unseen.)