Terminology index - a list of bike part names and cycling concepts (Cache)
There's a handy reference at the Park Tool Co. website, a bike repair map; it's a diagram of a bike with all the parts labeled, and is very handy! At the moment, the diagram is up at parktool.com/blog/repair-help. (They've changed the URL in the past, so this link may break.)
A road bike has the following parts (source):
A mountain bike has the following parts (source):
Edit: This page is meant to identify what things or concepts are (as per this thread in meta). If you want to recommend an accessory or a specific product you've found handy, please use the accessories page.