Tokyo is the capital of Japan. It offers multiple possibilities for trash recovering. In Tokyo freegans can dumpster dive by the supermarkets, convenience stores, fruits & vegetables stores, 100 yen stores; markets and streets in general are quite good for finding both food and other stuff.
Like in most places of Japan Tokyo residents must sort their trash in at least five categories. Everything has to be put into transparent plastic bags and left outside the building for garbage man to pick up. There are special days for specific types of things to be thrown away. It is an advantage for freegans since one can easily identify what is inside the bag without opening it and on what days specific type of trash can be expected.
Most supermarkets keep their garbage inside the building or put only bottles and cans outside and never bring fruits, vegetables or other expired food products out. Some supermarkets keep their trash outside only at day time and lock it inside after closing.
Some supermarkets throw away expired products in bulk, so it is recommended to check their trash regularly.
There are thousands of convenience stores in Tokyo but unfortunately they're not good to dumpster dive at all. The stores usually tend not to throw away (or they simply might not have much of) expired food, plus their trash generally is being kept inside - so don't expect too much from such chain stores as 7-Eleven, Lawson, etc.
Fruits & vegetables stores
You can try checking fruits and vegetables stores in your neighbourhood. Some of them leave no trash, but some do. You never know what you can get.
There are almost no small traditional markets in Tokyo. Existing ones are mainly wholesale markets and not everyone can enter them. Their trash is usually kept inside the locked territory which makes the dumpster diving very difficult or impossible. The biggest open market is Tsukiji and sufficient amount of food can be dumpster dived there.
Chez Lui in shimokitazawa puts out its leftover bread and pastries at night.
One can find 100yen store in almost every street in Tokyo. There are two types of them: the one that sells mixed kind of stuff, and the other one that sells mainly food. The latter one works non-stop, 24/7, and usually throws away more garbage than he first one. First thing you should do is to check where they place their garbage. Some of them lock it inside, some have dumpsters at the front of the store, and some place them somewhere nearby (behind the corner in the small street, etc.). Of course, you can't do anything if they lock their garbage in the store. If dumpsters are placed at the front of the store, checking what's inside them becomes quite complicated and probably not so convenient, especially when the streets are busy - you don't want to attract anyone's attention. But it's not so difficult to find 100yen stores that put their garbage in small streets around the corner - in such cases dumpster diving becomes really easy.
People in Tokyo get rid of things by leaving them on the streets for garbageman to pick them up. There are special days for specific types of things to be thrown away. For example, on a "paper" day one can find many books (e.g. manga comics), magazines, newspapers, etc; on a "burnable trash" day one can find clothes; and so on. You can also find bulky items (furniture, electric equipment, etc.) on random days left outside the houses, especially in residential areas. Some of them can be surprisingly in a very good condition. In this way, of course, you can find only non-edible things.
Freecycle group in Tokyo has more than 1000 members and is quite active (2-3 messages a day in December, 2008). The communication language is English.