Microsoft has said it plans to have "more than 1,000" ready when the phones become available, and to be "adding hundreds more each week through 2010".
Microsoft hasn't tried to match the numbers of its rivals: Apple announced last week that its iTunes App Store now has 300,000 applications for iOS smartphones and tablets; and Google announced that the Android Market had cracked the 100,000 number.
Instead, the company's focus has been on creating two groups of high-quality, visually impressive apps that showcase the strengths of the new OS: high-visibility applications, for example, apps created by companies such as Facebook and Twitter; and high-performance applications, including games, which demonstrate the platform's speed, smoothness, and power.
Mobile websites are already posting their Windows Phone 7 'best' lists, such as this group of 10 at Laptopmag.com, including Facebook, Bejeweled Live, Flixter, Yelp, and Star Wars: Battle for Hoth.
Application development for Windows Phone 7 is facilitated by the use of familiar, widely used Microsoft tools, all of which were early equipped with add-ons to support the rebuilt phone UI: Visual Basic, Silverlight, XNA Studio for games, and the Expression Blend application designer.
That ease of use is a critical element in Microsoft's strategy because existing applications written for the older Windows Mobile OS can not run on the new Windows Phones. The more C and C++ code these older applications have, the more has to be rewritten for Windows Phone 7. And for some developers, the current inability to use multitasking in the new OS is a showstopper.
Microsoft also has been reaching out to end users for app ideas. One contest offered a $5,000 grand prize, which was won by a single mother of four, who proposed a combination calendar and meal planner for families called "one tap organises everything". Microsoft has said it will have the free app ready by the end of 2010.
Along with the new tools, and in some cases, cash support or other financial incentives to attract some developers, Microsoft has reworked the online catalogue, both for developer submission and approval of applications and for user try-outs and purchases.
In doing so, Microsoft has tried to learn from mistakes made by its rivals.
Some number of Android developers, for example, continue to voice complaints about the online catalog's operation (for example, it relies on Google Checkout for payments instead of a popular scheme like PayPal), the difficulty in programming for diverse handset architectures, and the resistance of Android users to pay for apps. Last July, some developers launched an online plea, asking users to promise to spend $5 (£3.15) per week on Android software.
Another controversial Android issue is software piracy: until recently it's been relatively easy to to strip rudimentary copy protection from applications offered on the Android Market website, and then use, offer or even sell the software as your own. The problem isn't new, and Google has taken more aggressive steps in 2010 to make it harder to pirate Android apps.