Exclusively Ubuntu 10.04 LTS, managed with Landscape and Puppet. Normalizing the environment is crucial when managing a non-trivial amount of servers. LTS is ve

Exclusively Ubuntu 10.04 LTS, managed with Landscape and Puppet. Normalizing the environment is crucial when managing a non-trivial amount of servers. LTS is very nice when it comes to production servers, stable updates, stuff rarely breaks, it's universal and almost guaranteed to be supported by more exotic closed-source software. Upgrading to next LTS is usually painless, this hopefully goes for the upcoming version too.