There's this game online (Exodus 3000) where you have a base (Homestead) on Mars and you go around mining volcanos and ruins for Mars Dollars (MDs). If you pass by someone else's homestead you can attack them to win some of their MDs.
Anyhow, the point is that the MDs you collect have two uses:
- buy weapon, defense... upgrades
- convert to real cash at the end of every month
Now I'm doing it for the fun not the money, but it makes sense cuz the creators get paid by all the advertisements on the site.
Click this link if you wanna give it a try (and you'll be doing me a small favor along the way):
Have fun! :)