Microsoft has put forth major efforts to get developers on board Windows Phone 7 and it is obviously paying off with hundreds of fantastic apps in the Marketplace. I find new apps for my HD7 on a nearly daily basis and haven't found myself frustrated by a lack of apps. Games are the hottest selling apps on mobile devices and Microsoft's Xbox LIVE games are outstanding.
Microsoft is now accepting and certifying apps for Mango, which means these updated apps will have support for features like fast app switching, background audio, multiple and double-sided Live Tiles, better Search integration, and more. Since I have Mango on my HTC HD7, I will see these apps start appearing in the Marketplace. There is no word on exactly when Mango will be coming to devices, but with HTC likely announcing new Mango WP7 devices in September I imagine the update should be out next month.