For example, Lendle uses Amazon's API to allow you to lend out your Kindle e-books to other Kindle users. You have to be willing to lend your own e-books in order to borrow someone else's, but that's not a big restriction. Best of all, your book should be in the same condition when you get it back.
This removes one of the barriers to buying an e-book; I'm more likely to buy one if I know I can lend it to someone. This may not make authors terribly happy, but being found is a bigger problem for authors than piracy. Lending is a great way to get someone to try out a new author, and if they like it they'll become purchasers. There's nothing I love better than finding an author I like... it means I can collect everything that author has written and know I'll be getting something good.