Halo Custom Edition is a modified version of Combat Evolved that allows user created content to be shared and played. There's a multitude of maps and modes that range from the spectacular to the downright bizarre. See the fan made trailer above for reference.

How much does it cost?

$0.00 It's free to download and play, not to mention that the required specs to run it are really low. Refer to the guide above for installation instructions.

Thanks, I'll be checking it out.