MOUNTAIN VIEW, Calif. ( TheStreet) -- Google ( GOOG), ever a champion of doing business on the Internet, has opened the Google Apps Marketplace, a Webfront store that sells third-party business applications that can be integrated with Google's online office software such as Gmail and Google Docs.The marketplace has the potential to make life easier both for small businesses that develop software applications for other businesses as well as for customers looking for a cheaper alternative to traditional desktop software. For software developers, selling applications in the marketplace means tapping a customer base of the 25 million users -- and 2 million businesses and universities -- that already use Google Apps. The store launched with 50 applications, including a payroll app from software veteran Intuit ( INTU) and a spate of tools from startups, such as a project-management tool from Manymoon. Developers can make their existing cloud applications compatible with Google Apps using Google's application programming interface tools. There is a one-time fee of $100 to list applications in the marketplace, as opposed to the $99 annual fee that Apple ( AAPL) charges iPhone developers in the iStore. Initially, developers will use their own payment systems to sell apps on the Google Apps Marketplace. However, Google plans to add its own billing functionality to the site in the next quarter. At that point, the company will keep 20% of the revenue from customers who acquire new apps via the marketplace. Use of Google's APIs will remain free.