FreeBSD
http://www.freebsd.org/
FreeBSD originally targetted getting the best possible performance on the 386 platform. Lately, it's expanded to a number of other platforms including Alpha and SPARC, as well as to the heirs of i386, Intel's Itanium, and AMD's Opteron. Its primary goal is to be as robust and efficient as possible on its various platforms, both for server and desktop roles.
``The power to serve'': FreeBSD aims for high performance and ease of use by end users, and is a favourite of web content providers. It runs on PCs and Compaq's Alpha processors. The FreeBSD project has significantly more users than the other projects.
