SimCity now supports modded content, and Maxis has released a set of guidelines that potential modders must adhere to if they want to engage with the community. In a nutshell, Maxis don't want mods to interfere with "the integrity of our multiplayer experience", or "harm the experience of others". No tweaking of executable files, not copyright infringement, and nothing offensive or that infringes anyone's privacy. More details can be found here.