<p>Any good recommendations for a platform agnostic (i.e. Javascript) grid control/plugin that will accept pasted Excel data and can emit Excel-compliant clipboard data during a Copy?</p>

<p>I believe Excel data is formatted as CSV during "normal" clipboard operations.</p>

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<p><a href="http://dhtmlx.com/docs/products/dhtmlxGrid/" rel="noreferrer">dhtmlxGrid</a> looks promising, but the online demo's don't actually copy contents to my clipboard!</p>

Not an answer, but a warning: my company bought the 2007 Infragistics ASP.NET controls just for the Grid, and we regret that choice.
The quality of API is horrible (in our opinion at least), making it very hard to program against the grid (for example, inconsistent naming conventions, but this is just an inconvenience, we have complaints about the object model as well).
So I can't say that I know of a better option, I just know I will give a try to something else before paying for Infragistics products again (and the email support we got was horrible as well).