Safari is an online library of technical books published by O'Reilly and others. I discovered it a few weeks ago and I love it. It gives me cheap, instant access to a variety of books that I wanted access to but didn't want to necessarily purchase.
You pay a monthly fee to 'check out' a set of books (the number of books depends on how much you pay). You can't swap books around during the month (otherwise, everyone would just get the minimum subscription). The interface is all web-based, so you can access the material from anywhere. You can't share your account for obvious reasons, but they do offer corporate rates. The search feature is very nice too. Currently, they offer 326 titles. Minimum subscription rate is $9.99, but you can change your subscription rate from month-to-month or simply stop using the service whenever you wish, so there aren't any long-term commitments.