I'm in the planning stages of a major web site upgrade from classic ASP to ASP.NET/VS2005. Part of the project is improving our web site UI to give it more of a modern web application feel rather than the old style (ie late 90's) web site look and feel it has now. Therefore I'm evaluating various Treeview, Menu, and TabStrip style controls for ASP.NET to see how they'll fit into our plans.

So, I'm looking for your suggestions and recommendations on controls that provide this kind of functionality.

Price is an important consideration so any open source solutions would be nice followed by those that have very reasonable corporate wide licensing plans. However, another major consideration is that it is easy to use and understand. Others who'll be working on this project have limited ASP.NET experience and helping them be productive quickly is important. AJAX capability would be a plus as well.

Also, if you have any ideas on web application look and feel I'd be glad to hear about them since I have almost a blank slate at this point.