Running special applications-Superuser is an app that can only be run on a rooted Android phone. This allows you to control which apps have access to the "root" system. Another popular application that "rooting" affords is the ability to tether a computer to your Android phone so that the computer can access the Internet using the phone's data connection. Another program can allow your Android to be used as a WiFi Hotspot without having to pay your provider for the feature.
Freeing up memory-When you install an app on your phone, it is stored on the phone's memory. "Rooting" allows you to move installed applications to your SD card, thus freeing up system memory for additional files or apps.
Custom ROM's-This is the most powerful feature of "rooted" phones. There are hundreds of custom ROM's that can do anything from speeding up the processing speed of your phone to changing the entire look and feel of your phone.
Just search on google for the custom rom's there are thousand.. although didn't find any on XDA.