Here's a cool one. SilverStone has put together a very different sort of hard drive. It is called the HDDBoost (SlashGear) and (the company claims) it can boost HDD performance by up to 70%. It allows users to connect any 2.5" SSD to any 3.5" HDD. The drive creates a mirror of the front end data of the HDD and puts it on the SSD, which allows your computer to access those files much faster.
The best part about this drive? No special software is required. Just plug the drives in and the hardware takes care of everything automatically. The total price is around $50, although the device is only available in Japan right now. That should change if this thing really does work as advertised. It sounds like the perfect stop-gap between HDDs and SSDs.