tumblr is free, and popular. But not designed with comics in mind.
DeviantArt is also free, and also popular, but even less well-suited to comics.
Smackjeeves offers free membership, automates page upload, and has a community focused on making and reading comics. But it's not as popular. And personally, I hate the interface.
Drunk Duck is very much like Smackjeeves.
And Gaia's forum has Comic Creators if you're looking for feedback, and Promotions if you're not.