Google Books

Some of you may be well aware of this but google has been putting many, many books online. I have found this to be an incredible resource. Many of the books have only "limited previews," which generally means that you can read all but a few random pages. If you are thinking about purchasing a book, but want to get a feel of it first, this is a great place to go. Or if you are looking for some good resource material (like commentaries), it's a great place to look. Plus, you have the option of creating your own personal library. You can check out more here.