Twitter has started to advertise a number of applications using the small box on profile pages that it first introduced earlier this month. Before today Twitter had reserved the space for links to its own services like Search, but now they are advertising a number of sites and apps, including Tweetie, a popular iPhone client.

Other links popping up include Twittervision and ExecTweets, which was first announced earlier today.

The ads are unobtrusive, and they’re promoting useful applications that are all Twitter-related so they blend nicely with the page. The question now is how much money is actually changing hands – John Battelle writes that Federated Media is sharing some of its revenues from ExecTweets, but also notes that Twitter has a history of promoting apps it finds interesting without getting paid for it.

Update: Tweetie developer Loren Brichter says that he actually isn’t paying Twitter a cent to get featured on the site. Twitter came to him, explaining that it wants to promote projects like Tweetie which promote “variety, relevance, and value” (apparently a number of Twitter employees use the app).