Given that hybrid hard drives like the Momentus® XT never really took off (there is only one manufacturer, so tier 1 OEMs will not use them, plus current drives and only cached reads anyway), it looks like there are limited options for SSD hard drive caching, an add in SATA card or a software solution.
More recent drives like the Seagate 3rd generation solid state hybrid drive (SSHD) look more promising, but there are trade-offs. They can allegedly cache some writes, but at the moment they still have very little flash (8GB MLC, with a small portion of the NAND set aside for use in SLC mode, similar to SanDisk’s nCache) and they are only 5400rpm drives.
Unfortunately, none of the current options can make normal hard drives perform like SSD's. At best you get performance somewhere between SSD & HD and at worst you get performance which is even lower than the HD on it's own.
On the plus side, if you are upgrading an old SSD then using that old SSD to cache your large storage drive could be an excellent option. SSD caching seems to benefit much less from newer, faster SSDs so you get most of the caching benefit from older, relatively slow SSDs.
Add-in SATA cards
One option is an add-in SATA card which can provide this functionality. I believe there are enterprise level solutions for this, but as they are well out of my price range I haven't researched them. The HighPoint RocketHybrid 1220 is much more affordable though, and there is a nice article about this on Tom's Hardware Guide.
My experience with the Add-in SATA card option.
Having seen the Add-in SATA card option in action, I have to say that I'm not impressed with the Marvell 88SE9130 based card that I bought. HyperDuo was incredibly unreliable, the software kept crashing and the performance was rarely higher than the underlying hard drive on its own, even after hours of optimisation.
Even using the card as a 6Gbps SATA III port resulted in worse performance with an Adata S511 than just using the on-board 3Gbps SATA II ports.
Intel Smart Response Technology (may require a motherboard upgrade)
Another option is to upgrade to a motherboard with an Intel Z68 chipset, or newer chipset that supports Intel Smart Response Technology. Again, THG has a nice article providing an overview of what we can expect of the SSD caching capabilities of this chipset.
My experience with the Intel SRT option.
After having had the opportunity to play with a Z68 based motherboard, I was even less impressed with Intel Smart Response Technology than I was with HyperDuo!
At the time Smart Response Technology could only cache a Windows boot drive, so you couldn't have an SSD Boot drive and then use SRT to cache another drive in your system. You had to install Windows on the hard drive, install drivers, then the Intel® Rapid Storage Technology (RST) software and then add the SSD. At this point, if you were lucky then you would be able to see a new "Accelerate" tab on the RST application and if not then you may have to start the whole process from scratch to try and get it to work.
Apparently (thanks Nicholas) more recent versions of RST are substantially better. I haven't tried this yet, but apparently you can now cache a non boot hard drive as long as you start with a completely blank SSD (no partitions). You are still limited to 60GB of cache, but once the drive is set up as a cache, the remainder of the drive can be configured as a normal partition. Sadly you are still limited to caching a single drive or raid array.
Alternative software caching options
The last option is a non-Intel software caching solution, like Dataplex, which is bundled with the OCZ Synapse SSD. Unfortunately, this solution (like Intels SRT) currently only supports caching a single boot drive, so the only benefit is that it doesn't require a Z68 motherboard.
Unfortunately, I have yet to see a review from a site whose methodology I trust, so I can't tell how this option compares with SRT or the add-in card option.
Final option: Don't bother, just manage your storage manually
Fire and forget solutions are all well and good, but sometimes it's just easier to manage these things yourself. Use your SSD and hard drive as separate drives, put stuff that you don't need fast access to on the HD and stuff that do on your SSD. Move thisgs between than as and when you need to.
Tools like NTFS symbolic links, SSD Boost Manager and Folder2Junction could all help. See my answer to Trying to make SSD boot drive with Windows 7 and old drive is not accessible for more details.