ABEBOOKS.COM
ABEBOOKS lets you set up a free account with your used book want list. If they find the book, they send you email with the price, condition & other details.  You email back if you want it - but if the book is on lots of want lists, it is apt to be sold before you can email back.  It is hard to get a book off the want list though.

AMAZON.COM
Amazon lets you set up a free want list.  You also "pre-order" the book.  You list the maximum amount you are willing to pay for the book and authorize Amazon to charge your credit card if they find it.  The advantage is that no one can snatch the book up before you get an order in. I've gotten rare books for very reasonable prices this way.

BOOKFINDER.COM
No want lists, but I have better luck with this site than with many of the others.  Even if they don't have the book listed, I can often find bibliographic details for doing a better search - particularly nice when I only have a partial title or just the author's last name to go on.

EBAY.COM
eBay lets you set up free searches (acts like a want list) and will send you email when they find a match. Then you have to bid, of course, but rare books are often very cheap.