If you're not working on Windows, you might look into Tokyo Cabinet. It looks to be quite good. I'm waiting for the Win32 port, so I can't say much about it other than the performance looks to be impressive.
http://anyall.org/blog/2009/04/performance-comparison-keyvalue-stores-for-language-model-counts/
This is not an SQL derivative. It's a straight b-tree/hash based store. This is very welcome, in my opinion. The only competitive library that I'm aware of is Berkeley DB, which has less than desirable licensing.
