Which key/value store is the most promising/stable?

Question

I\'m looking to start using a key/value store for some side projects (mostly as a learning experience), but so many have popped up in the recent past that I\'ve got no idea

Answer 1

Cassandra seems to be popular.

Cassandra is in use at Digg, Facebook, Twitter, Reddit, Rackspace, Cloudkick, Cisco, SimpleGeo, Ooyala, OpenX, and more companies that have large, active data sets. The largest production cluster has over 100 TB of data in over 150 machines.