I just opened my email to see the picture of the mystery tool. Even though I couldn't see a very big view, I suspected I knew right away what it was. I checked in here and see that valve keeper tool has already been said. I still have 1 or 2 of them, though I don't use them so much anymore.
I have a few types of specialty valtrain tools, like flathead spring compressor, and smaller engine flathead spring compressor, and a couple more I can't recall. Also some specific-make tools for valve adjustment.