Having been doing a little C# development over the last few weeks, I was looking for an alternative to Visual Studio.NET (I really can't justify the expense at the moment), as such I found SharpDevelop.
#develop (short for SharpDevelop) is a free IDE for C# and VB.NET projects on Microsoft's .NET platform. It is open-source (GPL), and you can download both sourcecode and executables
It seems that the tool was developed as a case study in a Wrox book, and has now been released to the general public. Having played around with it for a while, I can say it looks pretty good. It is not as intuitive as something like Idea is for Java, but it is usable none the less, and a lot cheaper than the MS alternative!