Installing apps for Google Apps For Business via the marketplace has never been especially difficult, but now you have a second option. Google is adding apps to the Admin console as well.
Initially, 20 or so apps are being highlighted, though Google says it will expand the range in the future as additional apps are certified. To feature, an app has to use OAuth 2.0 and integrate with existing Google services.
Third-party apps: now easier to find and more secure [Google Enterprise Blog]