WARNING: This post was originally published in 2008 and hasn't been updated since.
The tips, techniques and technology explained here may be outdated. If you spot any errors, please let me know in the comments so I can adjust the article. Thanks!
Here's a very useful CLI cheat sheet for Linux, which will show most of your commands and simple usages. Probably not the easiest to use when you're not working on a native 1280*1024 resolution, but it doesn't take much to change that.