Userscripts.org

I think caching wouldn't be necessary if you just had used onmouseover event. That probably would be a lot faster than auto running it at once for all matching urls and at every 10000/counter miliseconds.

I agree with you, but in a common situation people would momentarily be interested to a few urls.
Or I just can't imagine a situation which people would like to know all extended/real urls.

You might understand my point after this draft.

// filtered_urls is an array of urls(anchors) that matches to whatever the pattern is

for (var ai = 0, at = filtered_urls.length ; ai < at ; ++ai )
{
	filtered_urls[i].addEventListener('onmouseover', function(e)
	{
		e.target.href = request_real_url(e.target);
	}, true);
}