FreeSO is an open source reimplementation of the engine used in The Sims Online, and private server. It was created as a fork of Project Dollhouse, as the simulator's code and emergent networking drove the project in a different direction.
It features a near full featured implementation of the SimAntics environment, which reliably runs all objects in the game. FreeSO currently supports online play using isolated "lot servers", which function without a "city server", but this is a temporary measure to allow users to play the game while it is in development.
Extra features of FreeSO include native support for resolutioms above 1024x768, GPU acceleration, 5 story lots, complex lighting, and 3D mode.
So far FreeSO remains the only successful reimplementation of The Sims Online.
FreeSO considers itself the "official server" and will not support any server based off of it, regardless of whether it's vanilla or not.