There seem to be so many validation frameworks around for .NET that if you plan to go with something other than the mainstream, there are literally dozens.
The most popular of the lot are the Microsoft Enterprise Library Validation Application Block, closely followed by the Spring.NET validation framework (part of Spring.NET Core) and the Castle validation framework.
I came across yet another validation framework today called the .NET Validation Framework (notice the capital 'V' and 'F'). They've made a beta release of version 2 in April 2008, but the project is still in development. The developer working away at the source code are Simon Cropp and Dane O' Connor (I assure you - he has nothing to do with the Terminator :-P). The project has been in development since April 2007.
I've been investigating the Spring.NET validation framework lately and they seem to have quite a bit of interesting stuff. Once you get the hang of it, you would find it more flexible than the Enterprise Library VAB, though the VAB has a smaller learning curve and suits most application needs.