I just read Old Man's War by John Scalzi, possibly the first whole book I've ever read on a computer (I've downloaded a number of books that I either abandoned after a few pages or didn't even start). It was most excellent, and I'll probably go hunt down the sequels from a library (there are three books, a free online novelette, and a new free online story, plus a fourth book coming out next month).
In case you haven't heard, Tor.com just launched as a weblog about science-fictiony stuff, and they're also posting short fiction. Another thing they're doing is posting a bunch of books (including Old Man's War) for free in a number of formats. They'll be freely available for download until Sunday.