jQuery is a really good place to start if you just want a little help with client side scripting for adding event handlers and using 'selectors' but don't want to go all the way with custom dialog boxes and other user interface elements.
ExtJs is often featured for its widgets. It can give you a cool Office 2007-like or Vista-like look and it's pretty comprehensive.
I haven't spent much time with Mochikit and Dojo, but I do find Mochikit's logging pane quite handy for client-side logging. It's for those times when you don't have access to a debugger (VS2008, in my case).