Book Burro
I know folks who read this blog are fans of real books also. If you are buying books online ( and who isn’t ) and if you use Firefox for your browser ( and you should ). You really need this extension. Book Burro detects when you are on a site looking at buying a book and goes out and hits other sites and pulls their prices down so you can compare. On install it compares Amazon, Alibris, Abebooks, half.com, buy.com, Barnes and Noble and a few others. Since I often buy books at Amazon and then kick myself for not checking Abebooks, it has saved me a few bucks a couple of times since installing.
It also is very subtle once installed so does not get in the way. Nice that is does not add a freakin toolbar, eating your browsing window or cluttering things more. Highly recommended.

