PC World: "So a fat pig jumps into my cab, and then he busts out into song while I carve through city streets. Nope, this isn't some deleted scene from the end of Slumdog Millionaire. I'm playing Karma Cab--a goofball mod for Half-Life 2.
What the heck is a mod? User-created game content--it's been around since long before YouTube ever made do-it-yourself entertainment a household concept. Old-school gaming nerds crafted game levels, weapons, character models--heck, entire games have been built using the guts of game engines for ages. (In fact, I highly recommend that you check out Mod DB for a huge selection of game mods.) That means lots of extra, free games derived from what you've already bought. The mod scene also happens to be a breeding ground for tomorrow's great game designers."