This is a simple calculator to compute the checksum field for the NMEA protocol. The checksum is simple, just an XOR of all the bytes between the $ and the * (not including the delimiters themselves), and written in hexadecimal.
For this to work you'll need to be using a browser that supports JavaScript and DHTML (most modern browsers do).
This page and the JavaScript it contains is in the public domain. Feel free to reuse it for any purpose.