Some awesome answers people!
Aurora: I've tried it, was interesting, but too buggy and badly documented for me. Interface was not just cumbersome (I play DF occasionally, thats OK), it used archaic Microsoft programming tools and a UI designed for huge screens. Bad combination on my computers.
Gearhead and Gearhead 2 are really fun. I remember those.
Battleships Forever is just mods, isn't it? No balance/physical laws involved in making stuffs.
I play Cortex Command -- but there's, again, really no "system" of customisation. Just script what you like, no matter whether of not it's practical.
Gary's Mod is cool too, with wiremod, but my computers are, as a rule, at least 5 years old and slightly upgraded. Not conducive to FPSs. Lag is a major problem for me.
OpenLieroX... Never heard of it. Judging from the post, it's mod based, but I'll check it out.
RA2 and Mindrover I love very much -- RA2 is showing its age and engine limitations (I'm looking at you lack of self-collision, spastic joints, and internal/external duality!. Mindrover would be so much fin with custom chassis in a multiplayer environment with some kind of resource harvesting or some such to make new bots.
RoboRumble... with a name like that and being mentioned in the same breath as RA2 and Mindrover, it sounds great! I should check that out!
Engine of War sounds like an awesome time waster! I loved bureau of steam engineering but found it simple -- this sounds like a step up!
Thanks for the input thus far!