There's also some companies doing SAAS apps for cities. I hope Detroit jumps ahead of other cities in using the cloud. Maybe even replace some of those aging computers with Chromebooks.
The one thought I have is that every city is guaranteed to have different laws regarding its charter/governing contract. That might dictate a lot of how that software interacts.
That being said, might be an area to make a killing having a highly-modular City CMS you can sell for half the price of contracted software.
The federal nature is a blessing and a curse, since though it's highly variable, you can sell this to one city without getting the rest of the nation to agree on it as well.