The VLC Media Player is an open source, cross-platform media client that supports the vast majority of media formats. Just check out the features page to see the list of them.
Firefox has built-in keyboard shortcuts for quickly navigating directly to the tab you want to get to. This is very useful when you are using Google Reader or Bloglines to read RSS feeds, and you’ve opened up a bunch of tabs. Before discovering this trick, I always used Ctrl+Tab to navigate back, or had to use the mouse.
If you have installed multiple versions of an application into your system, you may wonder which command is being run from the command line when you launch it. This is especially useful to figure out the path of php when running on a shared server, as many shared servers have both php4 and php5 installed.
Solaris includes a command line utility to get basic current system configuration information and output it to the console.