While using a low-capacity SSD to cache a hard drive has shown in some cases to be the next best thing to owning a large SSD as the proper primary drive, SSD caching is limited by the availability of the right technology. On the one hand, you have Intel offering it with its Smart Response Technology, on the other you have special storage enclosures such as HDDBoost from SilverStone. Smart Response Tech for now is limited to socket LGA1155 platform, and in it, Z68 Express chipset-based motherboards. In Japan, a new add-on card design has surfaced, by a company called Kuroutoshikou, which lets you use an mSATA SSD to cache a SATA hard drive, and its installation is claimed to be fairly straightforward.
The PCI-Express 2.0 x1 add-on card pictured below, makes use of a new Marvell-made 2-port SATA 6 Gb/s controller. One of its two ports is wired to an mSATA, the other to a standard SATA port. The SATA controller in this card features Marvell's HyperDuo technology, which works similar to Intel's Smart Response Technology, and installation is almost plug-and-install-driver. The card uses a half-height PCB, and packs a low-profile expansion bracket cover, if your SFF case demands it. Pretty much any make of mSATA SSD and SATA hard drives can be used. This addon card is priced in Japan for 3,980 Yen, which is about US $50.3, a $20 premium over some of the cheapest 2-port SATA 6 Gb/s cards are around that price range. It's possible that peripheral specialists of the likes of Rosewill, Siig, Syba, etc., might market such cards Stateside in the future.