Basically it's this game where you are meant to re-enact the assassination of JFK, but nobody ever does that, they just shoot fucking everybody. You have a score which nobody cares about, but the score is basically how accurate to the assassination you are. The world record score is 782/1000. If I'm not explaining it well enough maybe Wikipedia will do you good:
Here are the most useful mods:
JFK Revolutions V1.2: http://www.mediafire.com/?lvujhr5fsim
Adds a lot of funny stuff, like making the First Lady fat, and George Bush driving a bus.
Crowd + Bug Fixes: http://www.mediafire.com/?2tybqd0x2ym
Adds a crowd, gives everyone head gore, and fixes bugs.
JFK Reloaded Trainer V1.5: http://www.mediafire.com/?6dl4nn1tymy
A trainer which allows you to snipe from the grassy knoll and other useful stuff.
I would recommend you only use one of these mods at a time, since they all overwrite the core000.wad file so only one of them will work at a time. (I haven't tried the trainer, correct me if I am wrong with this.)