River View (New Town) - The Sims 3
While you can't currently begin a new town from scratch, and the Edit Town tool is limited by patches of uneditable terrain (these areas are not assigned to lots, so therefore cannot be altered), EA have released a new town called River View.
Downloading it is not as simple as it perhaps should be.
First, go to www.thesims3.com
Click the Create an Account link and register your game. Note that you will need your game's serial number to do this (those who have downloaded the game should have River View already).
You should then be given Sim Points to spend in the game's store, so head there using the navigational buttons on the website and download the River View map.
Download an extra town