Microsoft first released ASP (Active Server Pages) .NET in 2002 and positioned it as a framework for development of web applications and websites. One of its strongest points was the ability to use any of the .Net platform languages. It was also possible to combine server-side and client-side code, and even change a webpages's content using server-side code.
Websites such as stackoverflow.com, msn.com, microsoft.com, guru.com and many others make use of ASP .Net. Popular business applications Dynamics CRM, Sharepoint also use ASP. About 15% of the world's top 10000 websites make use of the Microsoft's technology. In total there are more than 40 million of such websites.
The upcoming release of ASP .NET vNext will be a major turning point in web applications development in C# and will bring the company to a significant milestone of releasing a technology built in a completely open-sourced way from scratch. It is also worth noting, that this time Microsoft decided to adopt currently trending web development libraries, instead of developing their own. Even their traditional development tool Visual Studio is no longer necessary to build applications.