A Shameless Plug
Listed thereon are links to over 250 complete comic books (or strips) that can be read online at no cost whatsoever. These aren't merely 4-page previews, but whole stories. Marvel alone has over 60, and Dark Horse has over 90 (though most of theirs are short strips). Even Crossgen still offers 20 full issues for you to read for free.
So if you've wanted to sample a new comic, or just enjoy free stuff, check it out. And if you know of any comics I'm missing (or if any of the existing links are bad; I've personally had some trouble with DC's PDFs), let me know.