http://www.marvel.com/digitalcomics/?page=11
You have to register and then you can view over 50 free Marvel comics..a cool mix of new (Nextwave1, Cap 18, Iron fist 1) and old (Kirby Thor, Romita Spidey, etc).
I know DC is offering free PDF downloads of a dozen or so vertigo titles too.
I guess the big two are finally waking up to the fact that its the 21st century...hopefully we'll see more and more of these.
I sampled Iron Fist 1 online and got hooked, same with Moon Knight (a few months ago)..digital comics dont cannibalize sales IMO..they are perfect marketing tools.