The most common option is to use IIS on Windows 2003 (Standard Endition) with hardware load balancers.
However you can write your own web server using .NET and the Http.sys object. See http://community.bartdesmet.net/blogs/bart/archive/2007/02/22/httplistener-for-dummies-a-simple-http-request-reflector.aspx for more details.
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