[sketerpot]

This one isn't just a cache; it's a database with a simple key-value data model. It has very good clustering support. It can handle datasets too large for memory. It's fast, allegedly easy to set up (I haven't tried it), and you can communicate with it using existing memcached client libraries. I think it sounds like an interesting option.

I also think that, in a few months, Redis will probably have the same nice characteristics of Membase, but with a much nicer set of datatypes. Membase does everything I described right now, though, which is worth something.