Apple's new Fusion Drive technology, introduced on its latest Mac models, is a hybrid drive that combines a fast SSD and traditional hard drive, along with smart storage handling by Mac OS X. You can create your own version at home using existing drives and the command line.
Mac developer Patrick Stein details the method he used to create a Fusion Drive using an internal 120GB SSD and an external 750GB HDD. With Terminal and the diskutil command, he created a single volume from the two drives. Surprisingly, the single volume shows Fusion Drive-like file handling, with no additional configuration. Stein notes that he won't be using the Fusion Drive, since HFS+ has an increased risk of data failure, but it demonstrates the possibilities.
If you'd like to create your own Fusion Drive, you'll need to run Mountain Lion. Any other combination of SSD and HDD should work OK, though. See the full details on Stein's Tumblr post.