After a long, hard semester of classes, manually calculating your GPA might be the last thing on your mind. Give your brain a rest and try some of these GPA calculators:
There are a lot of GPA calculators on the Web, but one of the better ones is from Koofers.com. The Koofers GPA calculator allows you to add up to eight courses and grades before tabulating your final GPA. It's also flexible enough to allow you to modify the GPA scale. If your school uses a unique scale, you can change it to match the one your school uses.
There are several GPA calculators in the Android Market, but none that really stand out. Though, there's an app called Class Buddy Lite, that allows you to add course details like, instructor, location, credit hours, time, notes, and of course, your grade. The GPA scale can be modified and Class Buddy Lite keeps track of your semester and cumulative GPAs. It's not a quick and dirty calculator, but it works and you might find the other features useful.
If you're more of a spreadsheet person, Office.Microsoft.com has a community member-written spreadsheet template that tracks every semester of your college career. The first sheet has a modifiable GPA scale and the second sheet tracks all your grades, with semester and cumulative GPA.
That's it. In case you actually do want to know how your GPA is calculated, take a look at the instructions from The University of Michigan.