This is an electric lego lightsabre I created for displays.

It will illuminate any colour of lightsabre blade for any Jedi/Sith character - or can be used with no blade at all as a working minifig flashlight, car headlight, a signal lamp that changes colour on command from the mindstorms RCX, etc etc. It is built in the spirit of lego; to be as flexible and useful as possible. 

But it definitely makes for a fun minifig display! 
(And using the lego wall adaptor means it can run 24 hours a day with no batteries to get used up :-)
My camera doesn't seem to respond very well to the red light (for vader's blade), so that photo doesn't look very good - I think it looks better in person.

The sabre has a thin black wire (out the side of its lower end) that typically dissapears between the cloak and body of the Jedi (and is even less visible against the black cloak of Darth Vader or Darth Maul). The connector at the other end of the wire is a Lego connector brick, and works with all the modern electric lego systems (mindstorms, trains, technic, etc).

The primary illumination colour is white, meaning that you can put in any colour of lightsabre blade, and it will light up. (This also means you can use a colourless blade and have a unique white lightsabre). If you place the connector brick the other way around on the power brick (ie reverse the polarity), then the sabre switches to bright red. I added this feature for two reasons: a) because the super-bright white light is a very cold white, meaning that it's spectra is strongest in blue, and weaker in red. We can't be having with a red blade being less bright than a blue one, so the combination of both red and white in the sabre means that all colours of light-sabre blade can be illuminated well. Reason (b) is that being able to set the colour state with a lego switchbox or mindstorms system makes the sabre more useful for other projects. All lego electric stuff does something when you reverse the polarity, so I couldn't have my sabre do less than that.

Suitable super-bright white LEDs of sufficient brightness and small size for this are very difficult to obtain - in fact I could find no suppliers that would sell them individually, the only way I could get them was to buy them in bulk, but I used them to modify my pocket mp3 player as well, so the purchase was worth it :-). 

I am impressed by the Lego electric systems - even though each item only does a simple thing, they are well engineered, and I have tried to do the same with this custom part.