In Second Life, the increasingly popular massively multiplayer online universe with free downloadable clients for Windows, Mac and Linux and free visitor accounts, players can build things. And since in-game currency is readily transferable to and from real U.S. currency, they are motivated to do so, with as many business models as there are in real life. (It's no surprise that most models are based on gambling.)

Of the many games and toys people have built in SL, one game qualifies as the ultimate runaway success: Tringo, a sort of multiplayer Tetris-like shape game, began as a Second Life franchise, and now not only dominates the virtual world, but is about to become a web game, a Gameboy Advance game, and (wha!) a TV game show.

Wagner James Au offers advice to would-be SL game designers on how to make a game successful in the virtual world.