I bought and read on my Sony PRS-600 books from:
1)
http://oreilly.com/ (plenty of software-development and computer related books)
Books are DRM-free, available (all formats for single price) as .epub, .pdf, .mobi and more (I used .epub on my sony and .pdf to print selected parts from PC). Note: they have nice "ebook of the day" initiative
2)
http://rosenfeldmedia.com (small but interesting choice of usability-related books)
DRM-free, .epub and .pdf (all formats for single price)
3)
http://www.packtpub.com/ (software-development, mostly open-source related)
DRM-free, .pdf (with my name and address in the footer on every page) and .epub (seemingly without such a thing). Both formats for single price but .epubs are not available immediately, at least not for fresh books.
4)
http://www.orbooks.com/ (small publisher, social, political, ...)
DRM-free, .epub, .pdf, .mobi, .prc
5)
http://www.sitepoint.com (mostly web design)
I bought a few (non-DRM) .pdfs from them (didn't work very well on reader due to difficulties while reflowing code samples), now I see they also offer .epub (which I haven't tested yet)
let me also add:
6)
http://pragprog.com/ (software development)
DRM-free .epub, .pdf and .mobi. I haven't bought from them yet so haven't tested, but I don't expect problems.