Machinima

Machinima (a combination of the words "machine" and "cinema") is a term used to refer to films made using a computer or video game engine.

Halo machinima
One of the most popular machinima series is Red vs. Blue, which utilizes the Halo, Halo 2, and Marathon engines.

Another Halo machinima series, This Spartan Life, differentiates itself from the competition by being a talk show, complete with a dance group (the Solid Gold Elite Dancers), a DJ (DJ Octobit), and guests. Part of the humor of This Spartan Life comes not from the makers of the show, but by regular players who appear and shoot at the crew, host, and guest of the show all the while unaware that a show is even taking place.

The Sims machinima
The Strangerhood is a machinima series created using The Sims 2 game. It is from the creators of Red vs. Blue.

Half-Life machinima
There are a number of Half-Life machinima made by their games or the popular Garry's Mod.