Seagate Momentus XT Solid State Hybrid Drive
The Seagate Momentus XT drive comes in 750 GB and 500 GB variations. Note that the 500 GB drive is older technology so it isn't nearly as good as the 750 GB version. The bigger drive sells for under $240 so the price isn't bad at all for the speed you get.
In this case the hybrid drive combines a traditional hard drive with a smaller cache made up of flash memory. It is the flash memory that gives it the big speed boost. Since there is a much smaller amount of flash memory than a dedicated SSD, the cost is a lot less.
Anytime you are dealing with an SSD drive, I'd suggest you use it for your operating system and your programs first and foremost. If you still have space left over, they you can use it for data. For those who want to add some bang to their system without spending as many bucks, this can be a very good choice.