Alright, I am slightly Biased with these types of things because I happen to be running a server from home that hosts said genre of gaming.
The Genre being Sandbox, but it does go under various other names.
One of the most common games in this Genre is Garry's Mod, a mod for Half-Life 2, which started as one mans playful modding, and 10 Major releases later got Steam so interested that it is now for sale at $10 a pop.
I have been following this game since V8 which is about when it started getting more popular exponentially.
And I have recently re-made my personal Gmod Server for the 6th time, and decided to create a Trailer for it in my spare time while I was bored.
Enjoy.