I have a Kindle and love it. I have had it, hmm, maybe 18 months?
Anyway just as an fyi, you can use Calibre (free program that you can download) to convert book types to the kindle type. You can actually use it to convert any way that you want to. Epub to mobi, pdf to epub, etc. I don't even know all the file types that it supports. It works pretty well for me.
Gutenberg.org has a TON of free books that you can download that are out of copyright and you can download them as mobi or epub and then use Calibre to convert it to the Kindle format.
Just thought I'd put that out there in case anyone didn't know about it.