The Park Tool GearClean Brush lets you brush, scrape, prod and pick your cassette, chain ring and derailleur jockey wheels to clean them to within an inch of their life.
Regularly cleaning and lubing your bike's chain reduces wear and improves shifting performance. Worn chains are bad karma. They drastically increase front and rear cog wear. Replacement drivetrains are expensive, so looking after the business end of your bike is well worth the effort.
I bought my Ground Effect chain brush a few weeks back and it has been brilliant. It's light, gets into the hard to reach bits and works a treat. Well worth the few bucks I paid amd a useful bit of kit