I remember a site with some games with Dan-Ball Powder Game physics + Mutilate-A-Doll physics including a fight simulation and a spider simulation, but i forgot the name of the site, someone knows it?
GMod:
Lua mods you can get from the click of a button or by just adding in a simple .lua to the manual scripts. (Mod Launcher)
Multiplayer. (TPTMP)
Everybody Edits:
2d multiplayer sandbox + you can use stkm for platformers + pixelart.
Y'all didn't think about non-fallingsand games did you?