It's entirely likely that if you've upgraded to a custom ROM you've got Android Terminal Emulator. If you only rooted, though, it's a must-have. Simple and free, it's a programming tool that allows you to run Linux utilities on your phone. What that means in practical terms is that you use this app to get rid of phone service provider bloatware. Droid X users, for example, have been blessed with a Blockbuster video app thanks to a deal that Verizon made. You can use Terminal Emulator to get rid of it, and others like it. And really, if it's your phone, why should you be made to suffer with their apps?
Terminal Emulator will work on an unrooted phone, but many of the best uses for the tool are unavailable without root access.