Places to Get Ebooks

Amazon specific search - This search removes all the public domain books and sorts price from low to high.
Amazon Special Offers - All the Kindle special offers on Amazon
Manybooks - Kindle format for downloads. Foreign language books as well. Has public domain books too.
Manybooks (Kindle edition) - Access through the Kindle web browser to download directly to Kindle. Choose Mobipocket format.
Feedbooks - Kindle format for downloads. Has public domain books too and I think others.
Project Gutenberg - Multiple languages. Seems to be in HTML or Text format (download text for the Kindle)
Free Kindle Books - a bunch of Project Gutenberg books in Kindle format.
Fictionwise - .mobi format. Some books free, some not, but some sales as well on the non-free ones.
World Public Library - all PDFs. Cost is $8.95/year. Over 400,000 classic titles.
Fictionpress - mostly original works, in Text format.

Conversion Software

Calibre - converts to/from a bunch of different formats including PDF and Kindle. Works on Windows, Mac, Linux.
MobiPocket - converts from PDF/Word/Text to .prc which can be read by the Kindle. Works on Windows.
- export stuff to Kindle format. Probably others, but not sure which. Works on Windows, Mac.