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C# and the .NET framework are fantastic, which is a bit of a strong statement from a committed Delphi developer, but it seriously is fantastic.
Developing ASP.NET sites is much easier than either straight ASP, or ASP + COM, and they run faster and are more reliable. What more could you ask for,
apart from good support on Linux.
Essential DotNet Stuff - download these IMMEDIATELY
NDoc is Nirvana for documentation writing. It can even create .chm files that are identical looking to the the official MS documentation by combining your XML comments and System.Reflection data. Its a MUST HAVE for all DotNet developers, and its free. Download it right now.
.Net implementation of JUnit. There is more about NUnit on the Testing page.
Using FxCop for the first time is a little scary because you realise how badly you've been writing code, but once you get the hang of FxCop it is fantastic and well worth the initial effort. I suggest you start with a small project and then slowly integerate FxCop into your daily coding routine.
Mono is an an Open Source implementation of the .NET Development Framework, targetted at the Linux environment and its been functional for quite some time - though they admit they have a long way to go to be fully .NET compatible.