Ubuntu is a Linux distribution based on Debian GNU/Linux. It has been fairly successful as a way for Windows® users to migrate to Linux. There are Live CD versions which allow later installation and versions meant only for installations.
Getting Ubuntu 
- Kubuntu, a desktop distribution using KDE rather than GNOME.
- Xubuntu (Ubuntu running Xfce,  designed for older computers - runs fine with 256MB of memory)
- Lubuntu uses even less RAM; LXDE
- Ubuntu Studio
- Installation on low memory systems 
Installation help 
Step by Step installation of Ubuntu.
- Ubuntu pocket guide
- UbuntuGuide.org -- the original Ubuntu Guide, since 2004
- KubuntuGuide -- the original Kubuntu Guide, with tips specific to Kubuntu
- Appnr - Install Ubuntu applications from the Internet
- Questions for Ubuntu
- Add/Remove.... to install Sun Java 6 Console and Sun Java 6 Web Start or follow 
-  Step-by-step guide for server installation