Linux Penguin
Linux is daunting to say the least. I've listed a few useful sites below. The best overall tip is to get stuck into the shell!

When X Dies: How to Kill and Restart X Windows: How to give you linux-box a vulcan neck grip.
Linux Forums:Useful source of answers for all distros.
XFCE: Xfce is a lightweight desktop environment for various *NIX systems. Designed for productivity, it loads and executes applications fast, while conserving system resources.
SourceForge.net: Create, Participate, Evaluate in a comprehensive range of open source software. You can access downloads, documentation, support and more through this powerful portal.
Best Open Source Project: A thorough run-down of the all-time greatest open-source applications.
A guide to compression/ uncompression: I always forget the command line syntax... here's a reminder.
Vi/Vim Cheatsheet: I usually give-up on vi pretty quickly (I'm very familiar with the :q command) so I need all the help I can get!

JBoss Getting Started Guide: Really helpful for getting rid of the dreaded Hypersonic and migrating to MySQL.