I`m in Tokyo
now, and have literally just got back from Akihabara, so decided to add to this node a bit.
Akihabara is geek heaven. No question about it. It has everything a geek could want in the fields of electronics and porn.
There are many consumer electronics shops with big "duty free" sights outside, which offer a good but not exceptional deal. The best stuff is off in the little side streets and on the upper floors of buildings. In these shops you can buy computer hardware, manga, character goods (dolls, posters, cards etc) and electronic components are very good prices. In particular, those geeks who like to roll their own when it comes to computers should be able to fund just about any component they need (even hard to find stuff like IDE encryption devices and water cooling equipment that normal shops just don`t sell (at least not in the UK.)) Many stored spill out onto the street, and once inside you will find them jam packed with an amazing array of stuff.
Keep an eye open for the little entrance ways, often with a flight of steps just inside, which lead to the shops on the upper floors of buildings. In these shops you can get the best deals, especially on used equipment. Compared with prices in the UK at least, 30,000 yen for 500MHz laptops with 12GB hard drives is a real bargain.
For electronics geeks and those into making quiet/modded computer, wonder through the electronic component alleyways. Here you can buy just about any component you need, from LEDs to resistors to ICs. Best of all, it`s all on show so you can get a good look at things like heatsinks and LEDs before you buy. Don`t forget to pick up a soldering iron, some copper board and a kilometer or two of wire while you are at it. Most of it is really cheap, for example massive heatsinks (good for passively cooled computers) can be had for 50 yen, or about 25 pence (UK pounds sterling).
Finally, remember to get a good strong bag from one of the computer places in the back streets. You`ll need it to keep all your newly purchased gear in!