My daughter sent me this site a couple weeks ago, when I was asking her if she knew how to do MLA bibliography citations. It's called EasyBib. It will do MLA6 and MLA7 citations for free. If you need APA or Chicago/Turabian style citations, MyBib Pro runs from $4.99/month up to $14.99 for an entire year. You can also get a free 1 week trial to MyBib Pro to see if it's something you really need.
Another thing that you can do automatically with MyBib Pro does is set up your footnote and parenthetical citations. If you are writing a master's or doctoral thesis, imagine how much time this could save.
The biggest time-saver, in my opinion, is the ability to auto-cite using a book's ISBN. This does not work with all books, but for those which it does, it is, to be cliche, the best thing since sliced bread.