This site seems an all-round best in class:
Lendle. The easiest, fastest, fairest, and best way to lend and borrow Kindle™ books.http://lendle.me/
You can log in, and you get two initial borrows. The more books you lend, the more you can borrow. I ran through their book list, and it is pretty great.
My only thought is - why didn't I think of this first?