Mmmm, booky, booky, booky-book

If, like me, you are a slavish devotee of the used-book trade, you'll enjoy Jessa Crispin's review of the three top online used book options: abebooks.com, alibris.com, and E-bay. Of course, nothing can beat browsing the musty stacks of your local used book shop, but when those fail you, or you need a specific title quickly, online is the way to go. I have to agree with Crispin that abebooks is hands-down the best of these, but I was surprised to see that she didn't include my personal downfall in the list: used books on Amazon.com. Like E-bay, many (though not all) of the Amazon.com sellers are individuals rather than bookstores, and the book ratings only sporadically follow the accepted grading system. Nonetheless, I've had excellent luck getting cheap, hard-to-find editions there. What are your favorite places to find used books online?