It's called abebooks.com and it's actually been around since 1996. (It's now a subsidiary of Amazon, but still run independently.)
Anyways, AbeBooks connects you to thousands of booksellers from around the world, allowing you to find virtually any book — even first editions, out-of-print and signed copies. Plus the prices are just fantastic. I found a copy of Charles Dickens' Bleak House for a dollar (or $2.73 with shipping). All of which means that the site is not only fantastic for book junkies, but also for folks looking for affordable gifts. (Anyone?)
Here's what AbeBooks says about itself: "AbeBooks is an online marketplace for books. Millions of new, used, rare, and out-of-print books are offered for sale through the AbeBooks websites from thousands of booksellers around the world. Readers can find bestsellers, collectors can find rare books, students can find new and used textbooks, and treasure hunters can find long-lost books."
You know what this means: One less excuse for why I have yet to read Bleak House. On the other hand, perhaps the holiday season calls for a different (and less tedious) Dickens classic, A Christmas Carol. AbeBooks, of course, is selling that too. And at $2.62, shipping included, it's quite a steal.