I have written before on mdcached - my not-so-pet-project I've been doing over the years, which I'm looking forward to bring to a really usable state in the coming weeks. I've cleaned it up a lot since March and I'm confident that the core of the system is correct and reasonably bug-free. I am renaming it to "Bullet Cache" but I am still calling it "alpha" code as it is not yet feature complete (some commands are missing). Anyway, it looks like due to the combination of advances in hardware and some algorithmical improvements, its performance is really good, close to 1M transactions per second.
I think that (aside from performance), the main thing about Bullet Cache is its use of cache record tags which allow mass expiry or mass retrieval of records - which is excellent news for caching complex data!
I am also looking for web design suggestions about the project's web site, as I don't think it's pretty enough :)
P.S. I promise I'll make the license more reasonable :)