Great game for communities, DayZ Mod is endless fun and it is one of the best group survival games out there.

From a community hosting stance: Dedicated servers have terrible optimization Lack of effort put in to any Linux Server Development Constant bugs with dedicated servers that they "promise" to fix.

Great game to play with friends, Competitive is very challenging and it has a great community platform.

Garry's Mod is one of the best games of all time. There are hundreds of gamemodes to choose from and will keep you entertained for hours upon hours.