Via The Third Rail: how many metro systems have you ridden in your lifetime? Go to this site and make a list. Here's mine:
From the top: Berlin U-bahn, Berlin S-bahn, Dallas, Guadalajara, Hanover, Kobe, Kyoto, London, New York City, Osaka, Prague, Dusseldorf Rhein/Ruhr, Tokyo TOEI, Tokyo Metropolitan, Vienna and Denver. I guess it's a start.
The list isn't exhaustive; they don't have a logo for Cologne or, for that matter, Houston. But new logos are being added all the time; the Denver RTD logo is at the end because it's a relatively new addition.
In a few years, they'll be able to add the logo for this system to their list!