In a move likely to accelerate the development and adoption of online business applications, Google on Tuesday evening launched the Google Apps Marketplace, an online store that third-party developers can use to sell their Web apps to administrators and users of both the free and paid versions of Google Apps.

The Google App Marketplace offers developers access to over 25 million Google Apps users, a group that includes some 2 million businesses.

Web apps that support OpenID-based single sign-on and OAuth-authorized access to Google Apps data can be made available to Google Apps administrators and installed in no more than four clicks. Once installed, third-party apps work just like native Google applications and are integrated with the menus in Google Apps.

"With administrator approval, [third-party apps] may interact with calendar, email, document and/or contact data to increase productivity," explains Chris Vander Mey, senior product manager for Google Apps in a blog post. "Administrators can manage the applications from the familiar Google Apps control panel, and employees can open them from within Google Apps."

Thanks to OpenID integration, explains Vander Mey, Google Apps users can access third-party applications without having to sign-in to each one. They also do not have to worry about updating them or manually syncing and sharing data, he says, resulting in improved productivity, a better user experience, and easier administration.

The price is a 20% share of application revenue from new customers, which must be handled through Google Checkout in the near future.

For the next few months, companies can continue to bill for Web app use on their own. Google is forgiving any revenue sharing obligation between March 9 and three months after it releases its Marketplace Billing APIs, due in the second quarter of this year. Expect to see the APIs released around Google's developer conference in May.