Akihabara is Tokyo's "Electric Town", located on the eastern side of the central Chiyoda ward. The area houses thousands of shops selling every technological gadget you can imagine, from computers to gaming consoles and vacuums to DVDs, at reasonable prices. This area is also known as the "Gamer's Mecca" and has in recent times become strongly identified with anime/manga (cartoon) subculture, with the legions of otaku geeks traipsing down on weekends known as Akiba-kei.
Akihabara is a major shopping area for electronic, computer, anime, and otaku goods, including new and used items. New items are mostly to be found on the main street, Chuodori, with many kinds of used items found in the back streets of Soto Kanda 3-chome. First-hand parts for PC-building are readily available from a variety of stores. Tools, electrical parts, wires, microsized cameras and similar items are found in the cramped passageways of Soto Kanda 1-chome (near the station). Foreign tourists tend to visit the big name shops like Laox or other speciality shops near the station, though there is more variety and lower prices at locales a little further away. Akihabara gained some fame through being home to one of the first stores devoted to personal robots and robotics.
Akihabara is also perhaps the last stronghold in the world for classic arcade gaming. Although arcades are still everywhere in Japan, and more so in Tokyo, the concentration (and skill of play) is especially high in Akihabara. The huge towers of Sega, Taito and others can't be missed, but places like "Hey Arcade" (on Chuo Dori) have entire floors dedicated to shooting games (think Galaga) and fighting games (think Tekken). Recommended for a nostalgic trip back to the eighties, and to check out the pros.
On a more classic note, Akihabara also has its own temple site, to the left off of Chuo Dori as you walk towards Ueno. It is mostly noteworthy for the festival it organizes around May. On this occasion, a massive shrine is traditionally carried through the streets of Akihabara, providing an interesting contrast with all the high-tech to say the least.
Sprawled in every direction off the main street Chuo-dori (中央 通り) are more smaller streets with even more electronics stores. Note that the further from the main street you get the better the prices - but the more you stray, the less foreigner-friendly the shops are. On Sunday afternoons, the main street is blocked to vehicle traffic and the area becomes a bit of a flea market - you can walk freely along the main avenue and many small vendors set up tables on the side streets. You can't miss the street performers; everything from maid-fetish karaoke to incan music can be heard on a good Sunday.
Though battery-powered electronics are basically the same world-wide, AC-powered electronics designed for the Japanese market use 100 volts, so "native" Japanese electronics may require a step-down transformer outside Japan. Even the US standard 110V voltage is too high for many devices. Also, these products have no international warranty, and (the kicker) are labeled and documented only in Japanese. Head for the many duty-free shops to find export models, which are priced at a premium though.
Larger shops can arrange sales tax exemptions for purchases of over \10000, saving you 5%. Unlike most countries, in Japan the tax is waived immediately, so there is no need to run through complicated reclaiming hoops. However, you will have to show your passport and (in theory) you should clear your purchase through customs at the airport on the way out. Many shops take credit cards, but some may charge you a small percentage added to the transaction. This is technically not allowed, and some credit card agents will reimburse the charge if properly reported.
Also keep an eye out for used electronics stores, which offer pre- owned computers, MP3 players, PDAs and other hardware at rock-bottom prices.
(Reference: wikipedia, wikitravel)