Customizable Games

Hello people Im looking for games that you can customize and make your own

For Example

There is Frets on Fire, where people make their own songs and post them
http://fretsonfire.wikidot.com/custom-songs

There is also Mugen where people make their own Fighting Characters and post them
I dont know where i can find a good website for this one
and Im not sure if Mugen is the best one out there

If u guys know of any others let me know

I played Mount & blade Berserk mod it wos even better then the original, but it must be very hard to mod

http://forums.taleworlds.com/index.php/topic,81326.msg2102700.html#msg2102700