Based on the lessons learnde from NUnit, XUnit was born to try to solve the problems encountered by NUnit as well as add many new features
There author explains the advantages of XUnit and its edge over NUnit here (http://jamesnewkirk.typepad.com/posts/2007/09/announcing-xuni.html)
Latest version can be downloaded here (http://www.codeplex.com/xunit)
Emad's Blog 