I like a good piece of Machinima, the creative use of the game worlds, characters and stories is often fun to see. The Trashmaster is perhaps the longest piece of Machinima I have ever come across at a full hour and a half. Built using GTA4 this is sweeping tale set in New York (Liberty) with and one tough sanitation worker who takes on the criminal elements. While I admit I havent watched the full thing from beginning to end there are some super examples of framing, camera angles and shot sequences. We get a movie soundtrack (several films represented) and a cool chase sequence at the end.
Not machinima, but still fun - The Princess Bride Chatty Duellists sequence with light sabres (LINK)
Also fun is the Half Life 2 sound conversion, using his own voice by Trase (LINK)
Or how about some WoW Music video parody (LINK)