The bounty added (Oct. 2014) asks about a specific page, and ads like it:
Here is example page http://pjrvs.com/a/advice the add appears if you mouse past the top of the page, then sets a cookie to prevent reappearing
To figure it out for this example, I first looked at the cookie that prevents it showing again and saw it's key name was "viewedOuibounceModal".
So that tells me that it's probably a script referred to as "ouibounce", used for producing modal windows.
I then viewed the source for the page and located (near the bottom) the scripting that's controlling the modal popup. The script refers to "ouibonuce", "ouimodal", etc. quite a bit, so by what we learned from the cookie this is obviously it. :)
Just above the script, is a DIV element named "ouibounce-modal" with a class of "ouimodal". It contains other DIVS with the links and text seen in the modal popup.
I fired up AdBlock Plus, and added a custom element filter of "##DIV#ouibounce-modal" to block all DIVs named "ouibounce-modal".
Now the modal popup doesn't come back (whether I have the cookie or not).