One of the best features of Google Apps is the Google Apps Marketplace. This is a clearinghouse of third-party applications that integrate with Google Apps, at a minimum leveraging single sign-on (either OAuth or, more commonly, OpenID) to access a variety of cloud-based applications with Google Apps credentials. Other applications like SocialWok bring much deeper integration of third-party software to many elements of Google Apps.
Not everyone shares my opinion, calling out the potential for turning a high-value, low-cost service into an expensive, Christmas-tree style set of disjointed apps, unified only by a Google ID. In fact, this is quite possible if administrators start adding applications with little thought to enterprise deployments simply because it’s so easy to just click the “Add it now” button when they find an app that looks interesting. And there are a lot of interesting apps, many of which are free or provided limited feature sets for free.
The average school or district sysadmin, looking at the Marketplace for the first time, will invariably begin imagining all the things he or she can do with integrated help desk applications, project management tools, graphics apps, and more. With so many of them free (and a whole lot of them being quite good), the temptation to just start installing is strong.
To help school staff sort out the increasing number of education-centric applications, however, Google is introducing an education category within the Marketplace today. Education is currently the only vertical that Google is targeting, primarily because of strong demand from education customers and the extraordinary popularity of Google Apps for Education. According to Google, the Apps Marketplace has over 250 business-centric apps that can often be made to work in educational settings. However, the new category will focus on apps that are designed specifically for education. Google also cited “Interest from education-specific developers for…more distribution and a centralized storefront.”
Google’s launch partners for this effort fall into three rough subcategories: learning management systems, productivity, and gradebook/SIS apps. Partners range from EduTone (which integrates Google Apps with on-premise LMS solutions like Moodle) to PlanbookEdu to BrainPop. While some big names are available in the Marketplace now (and even Blackboard is a target for a future app), perhaps the bigger news is the way this encourages developers in the education market to build and distribute their software through the Marketplace. Your SIS vendor, for example, could leverage the Apps API more easily now and integrate its services and software into Apps.
Like the Marketplace or not, the new EDU categorization of specific apps makes it very easy for both amateur and experienced school system administrators to add on major functionality with literally the click of a button.
We launched CourseDirector www.cdapp.info in the Marketpalace on March 2011 so missed being mentioned here. In terms of some feedback from a vendor interest has been really picking up in the marketplace now. Part of this is as a result of OpenClass, which despite their misleading announcement of a 'partnership' between them and Google (I noticed you wrote about that too) has really helped us find more schools to partner with, as it has brought more people in to the marketplace, some of whom prefer our LMS. 
