I frequent this place a lot. It is nice to try on a helmet for size and appearance, as well as goggles, jackets, etc. They also have stuff to help you maintain your bike like motorcycle jacks, oil, etc.
CV Carburetor Modifications
Excellent description of how to modify a stock carburetor to get better performance from your bike. Awesome pictures included.
Mustang Motorcycle Products, Inc.
This company makes great aftermarket seats. Very comfortable. Ask about them at your local Harley shop.
There is a kit available from here that makes it super easy to turn a Sportster into a chopper.
A lot of accessories for all types of bikes/riders.