A fair bit of models made for case closed awhile back by the Diamond group as well as 2 prominent modders.
You are Bob Dewey, Black Mesa janitor. You arrive at the research facility for another day at work: cleaning up the mess those smart ass scientists leave behind, refilling the soapboxes in the toilets, listening to Barney's stupid jokes and reading Penthouse Magazine. It's not the most exciting job in the world, but it pays good and you get lot's off free donuts. As usual, those scientists create black holes and open gates to other alien worlds filled with creatures who eat human flesh for lunch.
Nothing special, but this time, they really messed it up and those aliens found a way to our planet. And guess what, you are in the middle of all this. While Gorden Freeman kicks alien arse, Corporal Adrian Shephard fights Freeman and Barney Calhoun tries to save some scientists, you just try to save you own butt and get the hell out of Black Mesa.
It's a bigger mess you have to clean up this time...