Which key/value store is the most promising/stable?
Which do you recommend, and why?
I recommend Redis. Why? Continue reading!!
Which one is the fastest?
I can't say whether it's the fastest. But Redis is fast. It's fast becauseit holds all the data in RAM. Recently, virtual memory feature was added but still all the keys stay in main memory with only rarely used values being swapped to disk.
Which one is the most stable?
Again, since I have no direct experience with the other key-value stores I can't compare. However, Redis is being used in production by many web applications like GitHub and Instagram, among many others.
Which one is the easiest to set up and install?
Redis is fairly easy to setup. Grab the source and on a Linux box run make install. This yields redis-server binary that you could put it on your path and start it.
redis-server binds to port 6379 by default. Have a look at redis.conf that comes with the source for more configuration and setup options.
Which ones have bindings for Python and/or Ruby?
Redis has excellent Ruby and Python support.
In response to Xorlev's comment below: Memcached is just a simple key-value store. Redis supports complex data types like lists, sets and sorted sets and at the same time provides a simple interface to these data types.
There is also make 32bit that makes all pointers only 32-bits in size even on 64 bit machines. This saves considerable memory on machines with less than 4GB of RAM.
You need to understand what modern NoSQL phenomenon is about.
It is not about key-value storages. They've been available for decades (BerkeleyDB for example). Why all the fuss now ?
It is not about fancy document or object oriented schemas and overcoming "impedance mismatch". Proponents of these features have been touting them for years and they got nowhere.
It is simply about adressing 3 technical problems: automatic (for maintainers) and transparent (for application developers) failover, sharding and replication.Thus you should ignore any trendy products that do not deliver on this front. These include Redis, MongoDB, CouchDB etc. And concentrate on truly distributed solutions like cassandra, riak etc.
Otherwise you'll loose all the good stuff sql gives you (adhoc queries, Crystal Reports for your boss, third party tools and libraries) and get nothing in return.
At this year's PyCon, Jeremy Edberg of Reddit gave a talk:
http://pycon.blip.tv/file/3257303/
He said that Reddit uses PostGres as a key-value store, presumably with a simple 2-column table; according to his talk it had benchmarked faster than any other key-value store they had tried. And, of course, it's very mature.
Ultimately, OverClocked is right; your use case determines the best store. But RDMBSs have long been (ab)used as key-value stores, and they can be very fast, too.