Tokens & Farecards
Tokyo Subway Farecards

Tokyo has two subway systems which have semi-unified fare structures, but if you by a day subway pass, it is only good on the system you purchased it from. The other option is to get a Tokyo Metro Day pass which work on all subways, buses and JR trains in the Tokyo area. I prefer to purchase the Fare Cards which work on both subway systems and you don't need to keep buying single ride tickets.  

Tokyo now has the Passnet system which allows farecards to be used on virtually all the railways in the Tokyo area, except for JR

Don't forget to visit: Tokyo Area Transit Photos