Akihabara (sometimes shortened to "Akiba") has a strange conglomeration of dorky culture that is relatively unique (although similar areas exist in Osaka, for example).  There are several major themes in Akihabara:

Electronics are the main focus of Akihabara.  Although you can find a very large "Yodobashi Camera" directly outside of the station, as well as various "Sofmap" and other large chain stores around, the smaller stores is where the action is really at.  Those who want to buy common mass produced goods can buy them at any Yodobashi or Sofmap.  On the other hand, many of the shops in the area offer parts and kits.  You can find just the right color of LED, or modules to link your computer to anything imaginable. You can find kits to build robots, and vendors who can unlock any mobile phone on the planet.  You can find nifty devices like SD Card-to-Sony Memory Stick converters.  You can also find lots of spy equipment (Video camera pens, etc.), and lots of cheap generic parts and equipment.  Do you have an old laptop you can't find replacement batteries for anymore?  You'll find them at Akihabara.  It's like the electronics section of eBay or Yahoo Auctions, except you can browse in person.  You can also sell your used electronic goods you no longer need.  To quote one Japanese guy I know "I like electronics too, but those people in Akihabara, they are freaks!"  There is even a suitable show called "Akihabara @DEEP" which will show you how disturbed regular visitors can be.  

This is something even many Japanese haven't tried.  There are various cafes in Akihabara subscribing to the following model, the basic scenario is this:

1. You go in, and usually have to pay a minimum fee just to enter (something like US $4 - $7)

2. Some girl dressed something like a maid comes and takes your order.  Usually she says something like "Welcome home master".  As if I would have to pay $7 to enter my own home.  

3. They bring the (usually not so tasty, and somewhat overpriced) snacks/drinks that you ordered to you.  Often they draw a smiley face, kitty, etc. on them with ketchup or chocolate sauce.  

4. They do some sort of .... spell on the food "to make it delicious", usually involving repeating a magic phrase and doing cutesy movements to go along with it.  

5. You can usually pay (about $5) to have a photo taken with your "Maid".

No, they're not really maids, and they won't clean your room.  No, there isn't typically anything "Dirty" going on.  They don't even usually stay with you the whole time, as there are other customers.  Some of the shops have performance shows outside where the talent sing and/or dance to attract customers.  Most Japanese people I know have never been to a maid cafe.

This includes figurines of Anime and such, as well as transformer type toys, and more traditional hobbyist good like train sets.  You can find many shops specializing in models and modeling supplies, as well as comic books, etc.  

These abound in Akihabara, I don't know what else to say.  There are shops with DVDs, shops with "services", etc.

For further information, please see the Wikipedia article on Akihabara.