The process of creating these tools has been greatly simplified by the existence of similar tools for other EA titles. Games such as The Sims 3 and Spore use a remarkably similar internal structure, allowing the tools created for these other games (such as S3PE) to be easily ported to SimCity. If you’re a confident .NET programmer (I am, but I just don’t have the time); the open source nature of all these tools means you can join the development effort.