- Put adverts onto your website
- Ask for donations
- Offer to implement features
- Create virtual merchandise
- Sell "acheivements" and "player upgrades"
- Unlock features
- Sell merchandise
- Sell your community
- Provide an offline version of the game (for free) but require a small payment for anything other than token access (e.g. unrated friendlies) on a PvP server.
Some of these are similar and are along the lines of "allow players to pay for more features".
The main obstacle to all of these is having a game popular enough so that people will actually pay money. Suggestions for solving this problem on a postcard please...