Add-ons have been growing rapidly since it became practical to deliver extra content digitally (DLC: Downloadable Content). Most major gaming studios seem to be trying to work out a way to add this to every major release. Bethesda definitely had it for the Fallout 3 series (I don't know about earlier). The DLC can be pretty hit or miss depending on the game, but it almost always cost around $15 regardless of how much it adds. Mass Effect 3 actually had an add on on the disc already that you had to pay extra to play. DLC definitely has the ability to extend the life of a game you enjoy. I find it a hassle in multiplayer games where some friends have a DLC map pack and others don't. DLC can definitely feel like a cynical ploy to milk more money out of a game as well. "Game of the Year" editions of games generally include the main game plus all the DLC released if you pick the game up a year or two after release.