Arch Linux 2010.05 (x86-64)

Arch Linux is an independently developed, i686/x86-64 general-purpose GNU/Linux distribution that strives to provide the latest stable versions of most software by following a rolling-release model. The default installation is a minimal base system, configured by the user to only add what is purposely required, primarily meant for intermediate and advanced Linux users. Originally based on ideas from CRUX, a great distribution developed by Per Lidén.
Arch is fast, lightweight, flexible and simple. Those aren't very fancy buzzwords but they're all true. Arch is optimized for the i686 / x86_64 processors, so you get more for your cpu cycle. It's lightweight compared to RedHat et al, and its simple design makes it easy to extend and mold into whatever kind of system you're building.