I can't stand American comic books... they just... suck. Of course, manga's different, for reasons listed above.
Anyway, check out
http://www.manganews.net for unlicensed manga releases. And sites you'll want to go to are
http://www.snoopycool.com (of course I list myself first... who the hell wouldn't?),
http://www.toriyamaworld.com http://www.omanga.net, and
http://www.mangascreener.com
All of those provide some, if not all, of their archives in direct downloads.