For those of you who suck at "teh mathz" or are just lazy, this is the application (free) for you.
It is a bill tip calculator that alternatively gives you the option for splitting up the bill (including the tip). It comes in handy if you go out to eat and need to split up that huge bill 9 ways, etc.
The developers' website is located here:
http://fizzsoftware.com/webv4/billwm/bill.html and includes both Windows Mobile (Touch Screen) and Smart Phone applications. I'm including the cab (version 2.1) of the touch screen/windows mobile version below.