That's a very good question, and one which will take a while to list - But here I go!
There are many categories in ebay and naturally each one is popular and each one has it's best selling items or brands.  It would be nearly impossible for a mere mortal to get this information for himself - So what we have done here is collect it from a computer-ma-bob!  Simples!

The information supplied here is collected from the search requests that ebay get - Therefore they represent the items people are looking for the most - At least it allows you to configure your auction title with some really juicy keywords! (I DO NOT advocate using irrelavant keywords in your title or auction).

Thanks for your time, I hope I've been helpful!

SuperCooper

Right...  The categories and best sellers are as follows:-

CONSUMER ELECTRONICS    Mobile Phones & Portables
ipod
apple ipod
apple
ipod mini
sony ericsson
nokia
motorola
samsung
lg
treo 
 bose
 palm
 t-mobile
 camera phone
 world phone
 triband phone
 cell phone
 pda
 mp3 player
 mp3
 
 
   Top Brands
sony
dell
panasonic
pioneer
alpine
samsung
sharp
rockford fosgate
mtx
 xtant
 
 Home & Car AV 
 speakers
 calculator
 hdtv
 lcd tv
 car stereo
 satellite
 subwoofers
 gps
 television
 capacitor
 
 
Top CE Keywords
iPod
Bose
TV
speakers
Pioneer
Alpine
TiVo
Sony 
Sharp
Panasonic
 
 
    Home Video
DirectTV TiVo DVDs
15-19" LCD Flat-Panel TVs
20-37" LCD Flat-Panel TVs
DLP Rear Projection TVs
TV/DVD/VCR Combos
Sony DVD Recorders
InFocus Home Theater Projectors
Dell Home Theater Projectors
Sony, Sharp, Epson LCD Home Theater Projectors
 Hughes HD DirecTV Receivers
 
 
   Car Electronics
Sirius Satellite Radio
XM Satellite Radio
DVD/Video Units, In-Dash
Pioneer, Sony CD Receivers
Rockford Fosgate Car Amplifiers
CD/MP3 Receivers
12-inch Subwoofers
Remote Smart/Alarm Combos
6-1/2-inch Speakers