Spec# is an addition to C# to allow specifications to be explicitly added to methods. Specifications can state that a method takes a string (which must be non-nul), an integer (which must be between 0 and n), or an array (of which all entries must be non null). Previously in C#, this would have meant a fair few lines of code that manually checked parameters and threw the respective exceptions. Furthermore, without specifications, you cannot force any over-ridden method to comply to the same specification.
Spec# is currently a research project and can be downloaded at http://research.microsoft.com/specsharp/
It can be installed to run in Visual Studio 2003 and 2005. For both systems, it requires an application called Simplify. This is a java application that can be downloaded at HP Labs Java Programming Toolkit page. The zipped file that contains Simplify.exe can be found at http://www.hpl.hp.com/downloads/crl/jtk/download-escjava.html, but don't forget to first read the licence at http://www.hpl.hp.com/downloads/crl/jtk/agreement.html ;)