One of my favorite places in the world is Tokyo's Akihabara district. It loosely translates to "Electric Town" and is known for miles of electronic, gadget, computer shops, packed door-to-door and many stories high. If you're ever in this part of Japan be sure to set aside at least one day to peruse the shops, but leave early and wear comfortable shoes because you may find it hard to leave.
I've been to Tokyo four times and I'm careful to plan at least one day in "Aki" and possibly an extra, if there's enough time. There are so many shops that it's impossible to see them all in one day. Your best bet is to pick a theme and stick to those kind of stores, for example, Mac shops.If you're into older, retro, vintage Macs, you can also focus on them and stay in the used Mac district. Some people may just want to visit the arcades and videogame stores, or mobile phones shop, or straight consumer electronics. Plan your attack in advance, preferably with someone who knows the area, so that you don't become overwhelmed.
I haven't been to Aki in a few years but a good friend (Ole) visits often and sent me a care package of goodies that are convenient for any traveling Mac head. Pictured from top to bottom:
These are all great gadgets for travelers because they save space in your gear bag (the Japanese are big on retractable cables). What's the coolest thing you've ever seen in Aki?
Update: Akihabara News is a good site to follow for new gadgets coming out.
Update2: More information about where to buy the pictured items:
Ole picked up all of the above (except the Euro adapter) from the huge Yodobashi Camera in Akihabara.