A simple Tip Calculator.
It has presets for a 10%, 15% or 20% tip.
There's also a slider so you can choose your own custom tip percentage.
The custom tip percentage will be persisted between launches.

Now Chaz and I can calculate the tips for that cute waitress that used to work in Javas.
And Drew Bearymore for the female bar tenders, the tip will be on the receipt :P