Amazon has quietly launched book lending for Kindle ebooks. On participating titles, you can lend books to anyone for 14 days. Here's how it works:
To determine whether a title is lendable before buying, just check the product details section on the Amazon book listing. To check on books you've already bought, just point your browser to the Manage Your Kindle page, scroll down to the Your Orders section, and click the plus sign next to your books. Lendable books will display a Loan this book button (see the screenshot). Loaning the book is as simple as clicking the Loan button and filling out a form with the email address of the person you want to lend to.
Keep in mind that as long as the person has a Kindle, is running Windows/Mac, or has an iOS/Android/BlackBerry/Windows Phone 7 device, they can access your lended book. Very cool. Thanks Anthony!