I really like Baen, and buy a lot of stuff from them, simply because they "get it" - plenty of good and free content, no DRM on any of their eBooks, decent prices on the non-free stuff, and lots of format support. They keep track of what you have purchased, so if you go into the catalog, you will see an indicator that you already purchased a particular book, and you can download it again (in same or different format) if you need. You can read the free books online if you want, or if you want to check out a non-free book, they let you read the first 5-6 chapters to see if you want to get it - hooks you quick! You get to the end of the free chapters and say "I really want to find out what happens next", so you pay them your e$ and download the rest of it, saving a trip to B&N or Borders.