I would like to write an add-on for IE 8, but I want to use pure managed C#. Is this possible yet? I know for the longest time we were talking only C++.
Ultimately, the C# will end up calling into IE8's COM functions. There's a framework called Spicie that makes this easier, and some other examples here: http://www.enhanceie.com/ie/dev.asp
Generally, it's a bad idea to write browser extensions in .NET because there's a severe performance impact, and there's the possibility of runtime collisions because only one version of .NET can be loaded into a process currently; if two addons want to use conflicting .NET versions, one will fail.
Have a look at this
http://code.msdn.microsoft.com/SpicIE
This question is quite old now, but posting this just in case someone find it useful. =)
I have another answer here on SO about how to make IE 8 add-ins using C#:
How to get started with developing Internet Explorer extensions?