Yahoo (NSDQ: YHOO) continues to open its various properties to outside developers. The company announced Friday that users of Yahoo Mail, My Yahoo, Yahoo Connected TV and Zimbra would be able to access a number of new third-party apps starting this week. It’s the first time that outside apps have come to Zimbra, the company’s e-mail and collaboration software. All of the other services already featured (or were testing) some third-party offerings. Notable new Yahoo partners include YouTube, which Connected TV users will now be able to watch and search from their TVs.

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Inclusion on a Yahoo site can be quite a boon. Mint.com CEO Aaron Patzer, for instance, told paidContent that he expects that “if things go well” his site could add a “couple hundred thousand users, maybe more” due to the introduction of a Mint.com app that lets users track their spending trends from My Yahoo (Click on the thumbnail of the app above to see what it’s like). That’s a significant increase, considering that Mint currently has 1.2 million users.

Yahoo announced its “open” strategy a year ago, as part of an effort to make its properties more social (and keep users on its sites for longer). The most dramatic move in this direction is expected whenever the company gets around to relaunching Yahoo.com. Versions of the home page currently in testing show that outside applications from sites like the Wall Street Journal will be prominently featured there.