Amazon’s introduced a new feature for the Kindle (to compete with the Nook?): you can now lend out digital copies of your Kindle books to your friends & family (who else would you lend them to?).
Some restrictions apply. The lending period is 14 days and you can’t read your book while it’s on loan.
Here’s a great blog post that gives a good quick summary of the details:
And here’s a link to Amazon’s info page about Kindle lending.