Check out Facebook's FlashCache: they use SSDs as a cache on to traditional HDDs. You can have a read-cache to keep 'hot' data fast, and (if you want to) you c

Check out Facebook's FlashCache: they use SSDs as a cache on to traditional HDDs. You can have a read-cache to keep 'hot' data fast, and (if you want to) you can have a write cache that will buffer writes on SSD before flushing to HDD. I don't know whether it's ready for general (non-Facebook) use yet, but it is definitely one to watch.