I found an interesting entity validation library on CodePlex called EViL. The new Validation Application Block in the January 2007 CTP of Enterprise Library 3.0 also looks very promising. I just had a first look at both libraries and I like the simplicity and easy use of EViL, but I also like the power and flexibility of the Validation Application Block. It's possible to define validation in a separate configuration file that can be modified after deploying your application. I'm not saying that this would be a good thing without some major regression testing, but still, I like the approach of isolating validation rules. I will have a go on both libraries. If you know some other viable alternatives, please let me know.