I just discovered that there is a one day 'economy pass' for the 4 TOEI subway lines for the Y500 mentioned above. Given that some one way fares on TOEI lines are Y260-290, this can be a pretty good deal if you are going to places like the sumo stadium, Roppongi, or EdoTokyo museum (all on the TOEI Oedo line) or Asakusa (on the TOEI Asakusa line, natch!). Also, the TOEI Shinjuku line is pretty much a straight shot across Tokyo, if you happen to be at the right place to catch it.

This is the cheapest one day subway or train pass I've found in Tokyo, and while it obviously isn't good for everything, it is certainly usable for an entire good day of touring, especially if you are staying in the Shinjuku area where both the TOEI Shinjuku line and Oedo line have stations.