Non-rev seats.........
It's not a "non-rev" site, but if you go to Yahoo.com and click on the "travel" link....then click on "flights"....you can fill in the "To" and "From" as well as the "date" variables and this will pull up available flights. Then click on the "view seats" link by each flight listed and view how many seats are available. It doesn't work for all the airlines that are listed on the Yahoo.com site, but it does show seat availability diagrams on the majority of them. Go to Delta's web site to see their seat availability and I think American Airlines does the same on their website. I hope that this helps.
BBD