[Joykins]

I've used ebookfling and Books for Nooks.

I have over a hundred of ebookfling credits, and never bought a one because you get a credit every time you "instant fling" a book even if no one borrows it. I think I have been cycling the same 7 books for a few months now (someone eventually borrows them, but it may take a while). However, instant fling *downloads* are awesome because you know you will get them and get them right away. Their non-instant-fling loans are much more hit-or-miss. Their search features are dreadful as they do not allow filtering or sorting.

I also use Books for Nooks, which is a nook lending club, not an automated intermediary in the way ebookfling is. People post their books as available and you send a request through the site (which is tracked for acceptance and feedback, and also sent to personal emails) but lending is done on your own. There are good feedback mechanisms (you know how many books the person has borrowed, lent, and the borrower's ratings for all the loans, so you know how reliable the lender is. This works quite well, actually, and I have a much better request/loan ratio with the site--nearly perfect. They also have a forum/community aspect that ebookfling isn't trying for.

I've also done lending on goodreads. It's not a great way to track lending and make sure everything's equitable, but if you join a group for, say, a niche subgenre and people have lending shelves, it can work out well.