I never got into modding Jedi Outcast (though I used the heck out of the cheat codes as mentioned).
But I did mod Jedi Knight back in the day. My favorite level was a really well made Bespin level where you played as Luke dueling Darth Vader. You started on the landing platform by your x wing and followed the same sequence of events as the movie all the way to the carbon freeze chamber, out the window and onto the gantry. It was amazing and SO much fun!!