I've seen some people ask for a mod like this and a guild member of mine asked me to create it. When it was finished I decided to upload it to Curse just in case anyone in the UI community wanted to use it.
Nothing special, it just counts how many mobs you kill so if you are farming, it might sometimes be nice to see the number of kills you get. You can reset the counter and drag it anywhere on the screen you want. If you right click on it, it will show the options Start, Stop, and Reset. If you log out and back in it will remember how many kills until you reset it.