As a web designer and programmer I have a bookshelf that is weighty and a bank account that is inversely light. All books are expensive and technical books are quadruply so. Yesterday I bought a book that listed at $55 and was happy to get it with a discount coupon for $35. I was really just happy that they had it in stock!
I used to be a big fan of bookpool.com but they seem to have gone belly up for some reason. Their prices were competitive and their service was excellent. That's probably what killed them, they did it all from a base on Cape Cod and didn't outsource to Whackistan.
But now I have a new source.
Type in the ISBN and they come back with several sources of both new and used books. Instead of the book I bought yesterday I could have bought a nice used copy for $8! I know that Amazon has a similar service but I've never been comfortable with some of their practices. Give me an independent source every time.
This works for any kind of book, not just technical. Another great feature is that they list discount coupons that you can print out like a 30% off code at Borders Books.