Validators

• Validome.org

  Excellent markup validator

• W3C - markup Validator

  Markup validator from W3 consortium

• W3C - CSS Validator

  CSS validator from W3 consortium

Unix-like OS

• Debian GNU/Linux

  Debian GNU/Linux is a Linux distribution with strict adherence to the Unix and free software philosophy.

• Arch Linux

  Arch Linux is a lightweight and flexible rolling release Linux distribution following the KISS principle.

• FreeBSD

  FreeBSD is a unix-like and free operating system. It's descended from 386BSD and 4.4BSD.

• grml

  grml is a Debian unstable based Linux Live CD which contains sysadmin, network, security and data-recovery related tools.

Unix-like overall

• Distrowatch

  Unix-like (Unix, Linux, BSD etc.) distributions overview & listing

Webdesign & CSS

• CSS Zen garden

  Most beautiful CSS based websites

• CSS Beauty

  CSS design showcase

• The style works

  Demonstration of CSS (german)

Unix-like GUI

• Window Maker

  A very lightweight but highly configurable window manager (no desktop) for the X window system.

• KDE

  "K Desktop Environment" is a desktop environment for Unix-like systems and is based on the Qt toolkit.

• GNOME

  "GNOME" is another famous Desktop environment for Unix-like systems. It's based on the GTK+ toolkit.