Garry's Mod is a sandbox game with tons of community made mods. There are no goals in the game at all, it is meant more as a playground for ones imagination.
Ranked in these QuestionsQuestion Ranking
Pro Tons of community made add-ons allows the game to continually grow
Pro The games mechanics and mods allow for ridiculously funny gameplay
You can do some really crazy things in this game thanks to all of the community mods available as well as the built in, in game tools. Pretty much any scenario can play out in the game, if you so desire. Of course the winning combination of funny tends to be when players are just goofing around.
Con Previously free to download
Garry's Mod used to be a free download but it is now a paid game on the Steam store.
Con Tons of bugs
At least in earlier versions.