Debian Lenny 5.0.4
Debian is a free operating system (OS) for your computer.
An operating system is the set of basic programs and utilities that make your computer run. Debian uses the Linux kernel (the core of an operating system), but most of the basic OS tools come from the GNU project; hence the name GNU/Linux.
Debian GNU/Linux provides more than a pure OS: it comes with over 25113 packages, precompiled software bundled up in a nice format for easy installation on your machine.
See homepage for 64-bit version and older distributions.