Last night at Campfire One, Google announced the launch of the Google Apps Marketplace, a destination where developers can sell apps for integration with Google Apps. According to Ryan Boyd with the Google Apps Marketplace team, there are three steps for a Google Apps Marketplace app:
1) Have or create a cloud application and host it on the platform of your choice
2) Integrate your cloud app with Google Apps using available APIs
3) Create a manifest file describing your application and list it for sale on the Marketplace
You can find a whole series of videos from Campfire One, discussing the Google Apps Marketplace here at Google’s Developer YouTube channel. In addition, there is more information on the Official Google Blog, the Google Code Blog, and the Google Enterprise Blog.