
The Sims 2 PC - The Sims 2 is a great choice for creating machinima as you can film within the game. You can use various cheats and hacks so that you don't have to worry about keeping the sims happy or earning money. You can just get on and film.
You can also get The Sims 2 for the Mac.
The Sims University - The Sims 2 University is an expansion pack for The Sims 2, so you have to own The Sims 2 for it to run. The best thing about this expansion pack is that your sims can play musical instruments, fantastic for creating music videos. 
The Sims Nightlife - The Sims 2 Nightlife is an expansion pack for The Sims 2, so you have to own The Sims 2 for it to run. The best thing about this expansion pack is that your sims can now own cars. There are also DJs and karaoke as well as various other nightlife things, great for films and music videos. 
The Sims 2 Open for Business - The Sims 2 Open for Business is an expansion pack for The Sims 2, so you have to own The Sims 2 for it to run. The best thing about this expansion pack is that there are robots and various other new things to add to your films. 
The Movies - The Movies is now out! It contains everything you need to make fantastic machinima. You can even record the dialogue and the game will automatically lip synch the animated characters' lips. There are loads of scenes and characters to create drama, sci-fi, horror, westerns and much more. A community at themoviesplanet.com has already started to create custom content.
If you do not have an unlocking.ini file, create one.
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The Movies: Stunts & Effects Expansion Pack (PC DVD). You must have The Movies for this to work. Include incredible special effects in your films. From car crashes and changing weather to explosions, shattering glass and smoke, transform an ordinary action sequence into a cinematic showstopper. 
Machinima.com have put together a production kit. It includes everything you need to start making machinima with Quake 2, including a specially modified version of Quake 2 by Strange Company. 
Shadow of the Colossus (PS2). One of the few games that could be considered art in its own right, this is begging to be made into machinima. Gorgeous landscapes and fabulous animations. You can get footage without the hero by gaining the Cloak of Deception in Time Trial mode. This makes the hero invisible, meaning you can film panoramas, close-ups of the Colossi, eagle and hawk flights and fish. 
Jaws Unleashed (PC). Released July. Totally can't wait for this one. You play as the shark, so you can make your own Jaws movies from the point of view of the shark. 
Half Life 2 
Half life 2 X-box 
Quake 4 
Battlefield Vietnam 
Unreal Tournament 
Neverwinter Nights
Neverwinter Nights machinima tutorial
Quake 3 Arena 
Quake 3 Gold 
Grand Theft Auto: Vice City 
Software development kits for various games