Yesterday, Google launched the Google Apps Marketplace where users can discover and deploy third-party cloud applications that integrate with their existing Google Apps accounts.
From the Google Apps Marketplace homepage:
The Google Apps Marketplace offers products and services designed for Google users, including installable apps that integrate directly with Google Apps. Installable apps are easy to use because they include single sign-on, Google’s universal navigation, and some even include features that integrate with your domain’s data.
The Google Apps Marketplace Promo Video: