Ubuntu: Most "tip" widgets are pretty annoying and geared toward ultran00bs, but the Ubuntu Tips applet is actually pretty neat, giving out surprisingly good command line tips at a specified interval, so you can expand your Linux knowledge as you work.
Ubuntu Tips uses Ubuntu's built-in notification system to dole out small Linux tips every few minutes (or however long you want). It focuses on the command line, and even if you've been using Linux for awhile, there are probably some tips in there you don't know. I was pretty surprised at the quality of tips, and I ran across quite a few I didn't know. Of course, you can always start with our guide to time-saving Terminal shortcuts, too — but this applet is a pretty good supplement once you start working in the command line a bit more. Hit the link to check it out.
The Ubuntu Tips Applet is a free download for Ubuntu systems.