Microsoft’s Windows Phone Marketplace witnessed significant growth in March to push it past 12,000 applications.

Marketplace trackers WP7applist is currently tracking 12,143 apps in the U.S. Marketplace. The site explains 754 of them are inactive, 36% are available for free and the average price is $1.86 per paid application. Microsoft’s Windows Phone Marketplace is growing at an impressive rate. The software giant added 3,000 applications in March alone after reaching its 10,000 milestone earlier this month. WP7applist says that 145 apps are updated on average each day. In the last 24 hours, 112 apps were updated and 278 were added.

Luigi Violin, who founded WP7applist, says he expects the Marketplace to reach 30,000 applications in time for its birthday in October. “If I had to make an exact average of daily new releases, the Marketplace should hit 25K in the first days of October, but as the trend is slightly increasing I’d guess we’ll be well over 30K,” he said in an email statement to WinRumors. “It’s still growing at a nice rate, and even at a noticeable increasing rate lately so it really looks in full shape,” he added.

Windows Phone 7 launched on October 26 with 1,000 applications available. Five months later and the platform has increased over 10 fold. Microsoft appears to be building up a good momentum amongst application developers. Microsoft says it now has over 32,000 Windows Phone application developers, who deliver an average of 100 new apps per day. The Windows Phone Developer Tools have now been downloaded more than one million times.

Despite delays with its first platform update, “NoDo”, developers have a lot to look forward to this year. Microsoft’s new Nokia partnership is a great opportunity for new and existing developers as more and more consumers take advantage of Windows Phone. Nokia said recently that it plans to ship Windows Phone branded devices to developers when they are avaialble later this year. Nokia is currently engineering Windows Phone devices and is expected to wait for “Mango”, the next version of Windows Phone, until it unveils them later this year. “Mango”, the codename for what will likely be named Windows Phone 7.5, is currently in the early beta stages at Microsoft. The software giant revealed a number of new features in February that will ship as part of its Windows Phone point release later this year. Nokia is reportedly waiting for multitasking, IE9 mobile and others before it releases its first Windows Phone device.

Microsoft will also detail further developer related improvements to Windows Phone 7 during its MIX11 event in April next month. Joe Belfiore, who oversees Windows Phone Program Management, and is responsible for the design and software product definition of forthcoming generations of Windows phones will keynote MIX again. Belfiore will also be joined by Scott Guthrie, who heads Microsoft’s .NET Developer Platform. Guthrie runs the development teams responsible for delivering Silverlight, Visual Studio and the .NET Framework technologies for building client and Web applications. Belfiore is expected to further detail Microsoft’s “Mango” update and other developer improvements.

Microsoft has also promised further developer related platform announcements during May. WinRumors understands that the company plans to share a future roadmap for Windows Phone a special developers day on May 25 in London. Microsoft is describing the event as a peek at the future and what lies ahead for Windows Phone. “As well as new capabilities, we’ll examine any changes to the tools and SDK and explore what opportunities these might create,” says Microsoft. “At this stage we’re unable to elaborate any further but expect it to be a day packed-full of inspiring information.”