Since time immemorial, we've puzzled over the "guided" checkbox in the Marathon 2/Infinity physics file. Why can't we use it? Why was the SPNKR missle "guided" by default? No mattter. Now, you can use this Lua script to make any "guided" projectile that you shoot actively follow the player or monster that you're looking at. It can make for some messy netgames, let me tell you.
The Readme, as always, contains about 20 times more information.
Levels in map "misels":