Back to College: A website for an aggregate of information for adult students, they offer a search engine to help you find books drawing from a couple of websites. Their textbook page also provides links to a BUNCH of other online bookstores and price comparison websites.
BigWords: This one's great. They allow you to search for your book and draw information from a TON of different websites, finding you the best deal and giving you tips on ways to save even more money (such as membership benefit discounts if you sign up with your email on the seller's site.)
CampusBooks: This site is just like the rest. Nothing too special. EXCEPT for this interesting article I found for an app they have. Once you download the app, it looks for the textbook in your area. It will tell you if a local bookstore has it, or, even more interestingly, if your LIBRARY has it. I feel like the odds are slim, but if they do, you could be renting your textbook all semester for free. This is another reason to start looking now. I'm sure if it is available, it's going to go fast and the person who gets it will just renew it until the end of the semester, leaving you paying tons of money for something you could be getting for free.
Kindle does.) You will be able to access them on your PC (Windows XP, Vista & 7) or Mac (OS X,) but not on your nook. This may not be a problem for you if you take your laptop to class, anyways. If anything, it opens up Barnes and Noble's eTextbooks to even more users. But here's the problem: the reason these books cost so much is not the sole fault of your greedy university. Mostly it's the fault of greedy publishers. And while eTextbooks cut out the cost of raw material, shipping, manufacturers, and transportation, the price difference is not reflected in the cost of eTextbooks. Greed still rules, so the savings is truly negligible. It's only a few to ten dollars cheaper than the paper version, so I think I'm going to keep going used and save closer to around 50%.