The ASP.NET Web API has been released with the ASP.NET MVC4 beta release, which was released 2 days ago (14/02/2012).
You can download the ASP.NET MVC4 beta release, that includes the Web API, here:
Quoted directly from microsoft website:
ASP.NET MVC 4 also includes ASP.NET Web API, a framework for building and consuming HTTP services that can reach a broad range of clients including browsers, phones, and tablets. ASP.NET Web API is great for building services that follow the REST architectural style, plus it supports RPC patterns.
If you do not know what REST stands for and why it could be of any use, you can watch this 1h18m08s video on channel9 by Aaron Skonnard: http://channel9.msdn.com/Blogs/matthijs/Why-REST-by-Aaron-Skonnard
Considering how popular REST is these days, it might be interesting to cover the new Web API in this post. Originally we saw REST services coming up through the WCF pipeline, like the WCF Data Services or using a common WCF service with the WebHttpBinding, which works on HTTP verbs like GET, POST, PUT and DELETE. However WCF was created as a messaging platform, on which we are working with SOAP messages. The entire WCF pipeline is also optimized for messaging. REST services do work a bit differently, nor do they use any SOAP. Apparently Microsoft came to the conclusion that the integration of REST was not ideal with the WCF messaging pipeline so they moved the possibility to create REST services within the ASP.NET Platform.