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 where to begin. Just listing from memory, I can think of:

CouchDB MongoDB Riak Redis Tokyo Cabinet Berkeley DB Cassandra MemcacheDB

And I'm sure that there are more out there that have slipped through my search efforts. With all the information out there, it's hard to find solid comparisons between all of the competitors. My criteria and questions are:

(Most Important) Which do you recommend, and why? Which one is the fastest? Which one is the most stable? Which one is the easiest to set up and install? Which ones have bindings for Python and/or Ruby?

Edit:

So far it looks like Redis is the best solution, but that's only because I've gotten one solid response (from ardsrk). I'm looking for more answers like his, because they point me in the direction of useful, quantitative information. Which Key-Value store do you use, and why?

Edit 2:

If anyone has experience with CouchDB, Riak, or MongoDB, I'd love to hear your experiences with them (and even more so if you can offer a comparative analysis of several of them)