I have a similar requirement and decided to use the schema-less MongoDB.
MongoDB (from "humongous") is an open source, scalable, high-performance, schema-free, document-oriented database written in the C++ programming language. (Wikipedia)
Highlights:
- has rich query functionality (maybe the closest to SQL DBs)
- production ready (foursquare, sourceforge use it)
Lowdarks (stuff you need to understand, so you can use mongo correctly):
- no transactions (actually it has transactions but only on atomic operations)
- this stuff here: http://ethangunderson.com/blog/two-reasons-to-not-use-mongodb/
- durability .. mostly ACID related stuff
