One of the commonly asked questions on using .NET is the penetration of .NET Framework. This blog list the current statistics on .NET Framework penetration.
If you are considering using .NET Framework, you can do a sampling by analyzing the log files of your website. Internet Explorer reports the presence of .NET Framework on the client’s machine on its User-Agent Property. In my Windows XP SP2, IE 6.0 with .NET Framework 1.1, this is how this singature looks:
Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.1;+.NET+CLR+1.1.4322)
To successfully have this User-Agent Property recorded in the IIS log files of your website, ensure that W3C Extended Log File Format is selected and in the Extended Properties tab, the User-Agent option is selected.
Added on 13/Mar/2005: This blog entry by Yag from Visual Studio team lists the applications from Microsoft that use .NET Framework. Not a huge list, but certainly proves the bet of MSFT on .NET.