At the office we're designing a client-side MVC framework to compliment ASP.NET MVC framework. Fun stuff, but between both server and client side MVC, we're trying to figure out how we're going to build our tests.
On the server side, there are already some mocking frameworks out there. Some people swear by Rhino Mocks. Personally, I haven't even started doing the mocking thing, to my shame. But lately everyone seems to be pointing to 'Moq', http://code.google.com/p/moq/. This includes Phil Haack, who is a leader in the ASP.NET MVC development team.
What sells me on Moq already is that it's specifically targeting people like me -- people who know they need to mock their stuff to build tests, but haven't really gotten used to TDD, or who are overwhelmed by the whole process.
On the client side, I kinda pooh-poohed JsUnit already (but might still come back to it), but I just stumbled across WebAii, http://www.artoftest.com/webaiifxproduct.aspx .. this ought to be interesting. :) It's all client-side stuff but it's built around ASP.NET development (rather than Java, or Ruby, or specifically the Firefox browser), which sounds good.