[Freemium][1] - Make the bulk of your game available for free, but provide paid premium content (e.g. *Fantastic Contraption* and *Quake Live*). There's obviously a bit of a balancing act with choosing how much to give away for free: you have to make sure you give enough so that they can enjoy the game, but not enough to satiate their appetite.

[Adware][2] - Make your game completely free, but with adverts. Ads could be in-game, around the game canvas (for browser games), or could be shown intermittently throughout the game (like television ads). Note that the viability of this is debatable (*Quake Live* was originally ad-based, but turned to freemium when they realised that they weren't making enough money to cover running costs).

[Subscription based][3] - Players pay a regular subscription fee to play (e.g. most MMORPGs). This could also be complemented with an initial free period (e.g. first month free).

  [1]: http://en.wikipedia.org/wiki/Freemium
  [2]: http://en.wikipedia.org/wiki/Advertising-supported_software
  [3]: http://en.wikipedia.org/wiki/Subscription_business_model