Lots of games have downloadable content. In fact, DLC is becoming something of a point of contention between players and publishers. Indie game developer Ben Kane (who despite his moniker is not actually a comic book supervillain) has launched a game that doesn't just celebrate DLC, it bases its entire design around it.
Called DLC Quest, it's a satirical 2D platformer that imagines a world gone mad… for DLC! The game was created by Kane's own Going Loud Studios, and it's their third release on Xbox 360 after Zombie Accountant and Lair of the Evildoer.
In DLC Quest, players collect coins, which they spend on in-game DLC that allows them to do things like "move left" and "pause the game" and "have animation." And yes, there's horse armor. It's funny stuff, and the only think I can imagine that would be more annoying than an actual game built around DLC is if the latest novels had the same sorts of problems.