I have to say this: if you like FPSes, you're on a budget and you have a not-uber-powerful PC; the Orange Box is the best thing you could ever spend your money on. Its not just 5 great games in one but a whole lot more. There are a lot of other mods out there that can be a standalone game by their own rights. Here's a few: Age of Chivalry (warfare of the middle-ages), Insurgency (intense realistic urban combat), Zombie Panic (zombies vs. survivors) and much much more.
I was playing on a local server, one night and cant help but notice something. Every player was wearing a birthday hat. Everytime you blow someone up, their gibs (definition: chunks of meat) are replaced with present boxes, ribbons and other stuff you'd get for clobbering a pinata. Blood spatter? They got em replaced with balloons. I guess this is one of those parental control custom file mods, reducing the amount of in-game violence.