I've recently had the immense pleasure of playing Serious Sam, both the first and second encounters. The games were developed with both a serious sense of humor (look at me, I've started talking like them - seriously!

) and a good eye towards MODdability.
What with support for animated lights and textures, multi-gravity areas, teleportation nodes, fog, haze, hi-res textures (512x512, maybe even higher), etc etc (go download the demo for the Second Encounter already!!), it'd be cool and, with the talent we have here at HLP, easy, to do something FreeSpace-y with the game. The incredibly immense battles, with literally dozens of detailed monsters swarming the area at once, the humongous outdoor and indoor levels, the (hehehe) sweet weapons, ranging from a knife to a minigun to a genuine cannon-ball cannon - it's screaming for people to MOD it!
Anyways, check out the developers' website here:
http://www.croteam.com/I'd attach the SDK CHM Help File, but it's 20MB. One thing it says is:
Serious Engine implements game modification system. There are many types of modifications (MOD) that can be made to original game like: new levels, new playing modes, players, weapons and other models, items and enemies, new music and sounds, textures,.... Croteam encurages and supports MODs.
