At the Campfire One event last night, Google launched the Google Apps Marketplace and demonstrated how external Web applications from other vendors can be integrated into Gmail, Google Calendar, Google Docs, and other services that are part of the search giant's Web-based productivity suite.

In the quest for data liberation, Google's hosted Web services have long offered a wide range of APIs for third-party developers. With the launch of the new marketplace, however, Google Apps for domains is opening up even further and enabling external software to expose its own functionality directly through Google's Web-based applications. This will make it possible for third-party software in the cloud to offer broad interoperability with Google Apps and very tight integration.

When a Google Apps domain administrator installs an application from the new marketplace, it will be accessible to users directly through the Google Apps navigation bar, and the administrator will be able to configure it through the Google Apps control panel. Those are the simplest examples of how software can tie into the Google Apps interface. Google says that there are many other integration points that can be used by app developers.

In order to ensure that the experience is seamless, Google is relying on a number of increasingly important open standards. Single sign-on, for example, is facilitated by OpenID. Google Apps will act as an OpenID provider, and third-party Web applications that integrate with Google Apps will be implemented as OpenID relying parties. This will make it possible for users to access the integrated software without having to provide a separate set of credentials.

The new marketplace system uses OAuth to open up the user's data to third-party applications in a manner that is secure and transparent. During the app installation process, Domain administrators will be able to see a list of data access permissions that the app needs in order to operate. The applications will only be able to touch the user's data if they are given explicit permission by the domain administrator. Data access can be revoked at any time through the Google Apps control panel.

During the presentation at Campfire One, Google invited several of its marketplace partners to demo their new apps. Intuit showed how it has integrated its own Web-based payroll offering with Google Apps, allowing employees to access their paystubs directly through Google Calendar. Much deeper integration is also possible. Atlassian showed how its collaborative development tools can be woven into the Google Apps ecosystem, with interactive notifications, calendaring, and embeddable OpenSocial gadgets that can be snapped into Gmail, Google Calendar, and iGoogle.

I tested the new marketplace myself by installing the Aviary application on my gwibber.com domain. After installing the app, it became possible to access Aviary content directly through the navigation sidebar in Google Docs. Installation was a simple process that required only a few steps. The user experience wasn't flawless, however. Aviary had a bit of trouble handling the authentication token.

In order to participate in the marketplace, developers will have to pay an initial $100 entry fee. Google also takes 20 percent of the revenue from application sales. As the service is primarily geared towards business users, the marketplace is currently only available to Google Apps for domain users, not regular users of Google's services. For more details about the marketplace, see Google's official announcement.