Burger joints are one of the few cheap eat options in Tokyo.  It's also one of the safe heavens for those who are fed up with sushi and tempura.

McDonald's

McDonald's is quite ubiquitous in Tokyo.  Japanese refer to it as "Makku" or "Maku-do."  Its burgers might be slightly more expensive than they are in the US, but are still one of the cheapest option in Tokyo for light meals.  The special menu for Japan is "Ebi-puri burger" which looks similar to filet-o-fish but in fact it's a shrimp burger.  Also try the "Shea-ku" drink.  It tastes almost like ice cream but it's so hard to suck it through a straw.

Lotteria

It is owned by Korean retail/food giant Lotte.  The menu is quite similar to McDonalds.

Burger King 

The King is back!  Miami-based BK returned to Japan in November 07 and now has 6 locations: Shinjuku, Ikebukuro, Oimachi, Sugamo, Kanda, and Akiharbara helping those Whopper addicts get their "fix".   www.burgerkingjapan.co.jp

Wendy's 

There are a few Wendy's in Tokyo.  Mostly the same menu as in the U.S.

Mos Burger 

Mos Burger is a Japanese original great burger restaurant.  It's slightly more expensive than others, but its burgers are superb.  Every burger is made to order.  You might want to try its "rice" burgers: sauteed beef is sandwiched by rice-made buns.  It's not so tasty compared to other burgers by Mos, unfortunately.