I'm surprised nobody has suggested this yet.
[hider=Garry's mod]
[b]Pros:[/b]
Collaborative- Everyone in the RP can contribute
Interactive- Everything is moveable and you can literally jump, walk, and etc. all around it
Customization- Tons of 3d props to choose from along with the ability to change prop's color and properties. Tools allow props to be given special capabilities and/or functions
Easy to use- A few tutorials or hours of fucking around and you'll be making bases in no time
It is just really fun
[b]Cons:[/b]
Cost- Not that Garry's mod is expensive. But it aint free. 
[/hider]