A free GPL IDE for MicrosoftDotNet: http://www.icsharpcode.net/OpenSource/SD/

I've used it a bit and it's an impressive free IDE. Its forms designer is very good. Its code editors for both CsharpLanguage and VB.Net are very useful. It even includes an integrated unit testing framework. Unfortunately, theirs is a slavish copy of JUnit without the added .net-friendly features of NunitFramework. I've had trouble in limited use getting it to recognize external assemblies like NUnit's. I'm sure it's possible and that it's some kind of "Well, duh..." trick that I'm not seeing. But the problem does highlight the paucity of documentation. But this is still a beta in Oct. 2002 -- and a remarkable one at that.

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Since Oct. 2002, a number of things have changed - #Unit had been refactored long before that date actually to support projects spanning a number of directories - the Forms Designer has made quite some advances, the integrated help browser has been improved and a port to Linux is in its very early stages. There also is a wiki now dealing with #develop related topics: http://www.icsharpcode.net/heyhey/wiki/