Membase is another "NoSQL" offering that looks very interesting. It uses the same simple key/value store interface as Memcached, but unlike Memcached it offers permanent storage on disc rather than just in-memory caching. The benefit of offering the same interface however is huge: all of the software projects out there already using Memcached are well placed to migrate away from relational databases like MySQL for permanent persistence behind Memcached, and drop in Membase instead.

There is an excellent and clear interview with James Phillips of Membase on FLOSS Weekly here:

FLOSS Weekly 141: Membase

In it James discusses how, amongst others, Zynga as the developers of the hugely successful "Farmville" and "Mafia Wars" applications on Facebook switched to Membase to replace MySQL on all of the traffic-heavy applications. You can find out more about Membase itself here:

"Membase.com" - http://www.membase.com/

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Updated 2022 : note that the above post was originally published in 2010, but is left here for archival purposes. Sadly the Membase project is now dead, so I have unlinked their homepage which is now offline.