I've used
Scribd,
Smashwords and
Feedbooks.
General impressions:
- Scribd is simple. Create account, upload document, share with world. On the other hand, the interface is halfway between social network and fileshare, and I find it... irritating, I guess. You can sell content IF you're in the US.
- Smashwords is fairly easy to get going with, although to get a GOOD looking ebook you need to follow the formatting guide scrupulously. You can sell or offer ebooks for free, regardless of where in the world you are (Paypal acct required for international payments). It'll take a .doc or .rtf and port it to the various formats for you. Images can be... problematic. Offers the most formats.
- Feedbooks produces the best 'conversion' due to making you cut and paste each chapter/section of your book into it separately. Bit of a pain to do, but it does give the best-looking output. Offers the popular formats - epub, mobi and pdf.
Does that help?