There are a lot of unofficial clients available for WolframAlpha in the Windows Phone Store, but now the official app is here. Available for $2.99, WolframAlpha uses a computational engine to deliver results across a variety of domains.
The domains include Mathematics, Statistics and Analysis, Physics, Chemistry, Materials, Engineering and many more (seriously, the list goes on and on and on). While there is a lot of functionality on offer, file upload and personal analytics are not yet available.
Download WolframAlpha from the link below, and let us know what questions are you looking forward to asking the service's computation engine.
Update: Also, the Windows 8.1 version of WolframAlpha is now available. The $2.99 for the app covers both versions.
Thanks for the tip Chris!