The [link|http://www.microsoft.com/downloads/details.aspx?FamilyId=9B3A2CA6-3647-4070-9F41-A333C6B9181D&displaylang=en|.NET Framework SDK] is free and includes the complilers.
If you are looking at ASP.net development, [link|http://www.asp.net/webmatrix/default.aspx?tabIndex=4&tabId=46|ASP.NET Web Matrix] is a free IDE.
For WinForms development you might try [link|http://www.icsharpcode.net/OpenSource/SD/Default.aspx|SharpDevelop]