The best client-side reference I've seen is Microsoft's own DHTML reference.
Head over to http://msdn.microsoft.com/en-us/library/ms533050(VS.85).aspx
If you click on DHTML objects, it will list all objects available for scripting. If you click on a certain object, for example select (<select>), you will get a list of all available attributes, events and methods. Very handy!
Only thing is that there is no mapping between server control and client object. But if you know your html, it's not hard to do this mapping yourself :)
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