I hear great things about Tor Books from a few Science Fiction authors who use them as their publisher. When they launched Tor.com with their newsletter that included a link to a free eBook, I sighed up right away.
As much as I would like to be reading more fiction, I haven't been able to keep up. In fact, I've only read part-way through the first book, and I've already downloaded 5 books as part of their newsletters. I now feel guilty downloading when I might never have the chance to read.