This is a great little summary article of most of the non-relational data store paradigms around and some good links to examples of the software that implement them:

Relational databases, such as MySQL, PostgreSQL and various commercial products, have served us well for many years. Lately, however, there has been a lot of discussion on whether the relational model is reaching the end of its life-span, and what may come after it.

Should you care? Which database technology should you be using?