Zumobi, the startup with a widget platform for mobile devices, said its eponymous software is out of beta tests and is now available for public downloading.

The company is a spinoff from Microsoft and was formerly called ZenZui. Its plan is to give away the widget platform but support it with banner advertisements and share the revenue with developers.

Ben Bederson, chief scientist and co-founder, said that instead of scrolling through widgets a la Macintosh, Zumobians will focus into and out of the miniature applications by using a so-called zooming interface which will initially hold 16 programs. A popular example of such an interface is Google Maps, in which users can pan in a wide view and then zoom down into detailed views.

Zumobi will be bundled into the next release of the Windows Mobile operating system because of the company’s historical ties, Bederson explained. That release is likely to be Windows Mobile 6.1, expected this spring or summer. Microsoft has a minor investment in Zumobi and declined to comment.

Features of Zumobi added and fixed based on the beta test include a smaller installation file, a simplified program exit method, the ability to rate widgets (known in Zumobi-speak as tiles) and installation directly from a WAP browser.

Next up for the Seattle company is a J2ME version that will run on Research In Motion’s BlackBerry devices. Bederson said he believes the Apple iPhone is currently the pinnacle of mobile interfaces, and is a customer himself.

“Phones have just been way too complicated. The challenge with traditional GUIs is they’re a general-purpose solution. So I think the challenge is to try and figure out how to make the actions that you do very frequently, very simple,” he said.  “Some phones have done that well. Currently, Windows Mobile, I think, gets in the way of your everyday activities.”