Someone got a python script for blender 2.53 that makes your character jump, I had one but it was allot of bugs in it And dunno really how to fix it.
This is qite a general question. Without details I would say: no script needed. Just apply an upward force to the character.
Always when i have done that i get the feeling that it looks so unnatural. So im not using it But i think i have managed a stable script. Thx anyway