Binary Fortress Software's ASP.NET ViewState Helper, easy to use
http://www.binaryfortress.com/aspnet-viewstate-helper/
Nikhil Kothari's Web Development Helper, need to read manual before setup, not so clear-cut
http://projects.nikhilk.net/Projects/WebDevHelper.aspx
FireFox's Web Developer Toolbar, while not specific to ViewState, can help you analyze a page.
https://addons.mozilla.org/en-US/firefox/addon/60