Suckless: Suckless is a group of developers producing very simple, very functional programs for users not afraid of using a keyboard.
OpenBSD: OpenBSD is a minimalist, secure BSD-derived operating system, great router platform.
Slackware: Slackware is a Linux distribution which tries to stick to UNIX(r) philosophy, exceptionally minimal.
Arch Linux: Arch is a Linux distribution which is also exceptionally minimal, great binary-based distribution for technical users.
Gentoo: Gentoo is a source-based Linux distribution, USE flags are great and you can't beat the ability to quickly assemble packages with Ebuilds.