This is a VERY fun little (not so little) sandbox game with addons from the very large community. This game is probably one of the best games ever.

A cartoon-looking first person shooter made by Valve. Although, it is quite difficult to go about Team Fortress 2 without purchasing anything. e.g. MVM

A fun little game of gun play made by Valve. Competetive is fun with friends, and very addictive.