I believe using Blackberry gives you PRC or MOBI, whatever, they both work on the Kindle. Don't forget the mobireader site. There are also Mobilereference books there. Again, buying entire collections is a huge savings if you like the classic authors even more so when you just break down and get packets containing multi colections. I couldn't read Edgar Rice Burroughs any other way (Martian series, Tarzan series, Pellucidar series, Caspak series, Mucker series....).
In a lot of cases Amazon just takes Mobipocket freebooks and make them down-loadable to your Kindle. You will recognize the cover picture. They currently are having a sale 50% off the big collection packet.
http://www.mobipocket.com/freebooks/default.aspx