Google’s certainly made some enterprise inroads with Google Apps, and now it’s opened the door for other cloud-based service providers to build on that success: it’s launching Google Apps Marketplace to sell third-party web apps that integrate with the Apps suite. The apps are sold as a subscription, with both monthly and annual pricing, and the billing is all handled by Google. Since it’s all targeted at the enterprise, the apps themselves are pretty dry — we’re talking notables like Intuit Online Payroll, eFax, and TripIt — but it’s pretty easy to see how Google could build a similar consumer-level marketplace into Gmail and Google Calendar sometime in the future. And then? Skynet. Video after the break.