Machinima
Machinima is a term used to refer mainly to videos with a purpose made in video games. There is a firm distinction between a machinima and an ordinary video of gameplay, normally being that machinima is associated with acting, and game characters talking to each other rather than fighting. One of the most well-known examples of a machinima is Red vs Blue, a series created in Halo 2 (originally in Halo 1), and distributed freely on the internet.
Machinima has slowly become a widely popular form of video, and many games are used to create it, including Half-Life 2. (See more in Machinima in Source. Many examples of machinima videos can be found at websites such as Machinima.com.
Legal issues
Technically, the materials and models used in creating machinima videos belong to the game's creator, so they cannot be published with a price tag, and it is debatable whether most machinima violates copyright laws. Red vs Blue, for instance, was contacted by Bungie, the developers of Halo, in their early years, but thankfully Bungie was willing to work with them to divide profits. Red vs Blue has now extended its presence to Xbox Live, and is known worldwide.