Here's is my list of Web sites that I check every semester...I hit everyone one with my list of ISBN's. I've also grabbed a few of the sites posted here in this thread and added them to compile the whole list.
I recommend you click each link and add it to your favorites. Put them all under one folder called BookSearch (or something to that affect/effect) so that you can find them next semester/quarter.
AddAll Book Search [addall.com]
Amazon - Of Course [amazon.com]
I like that last one...to bad they don't have the college I'm going to in their database...I like the idea of what they've got though.
If your state college is like mine...they use Efollett to list books required for upcoming semester classes. I use this before school starts to get the book title and start looking for an ISBN and the above web sites to find my books. Efollett Web Site [bkstr.com]
Hope this list helps someone else out!