BundleRabbit is a DIY ebook bundling service. Curators can browse through thousands of ebooks, combined the ebooks into a box set, and then sell the box sets.
BundleRabbit also offers collaborative publishing. This allows co-authors to publish a book (both ebook and print) and not worry about splitting the royalties ... BundleRabbit handles the split automatically.
The BundleRabbit service has pretty much taken over all my free time. Because of this, the other websites listed below haven't been updated in a while.