What is the most useful accessory you got for your bike? By useful, I mean something that improved the enjoyment or functionality of using your bike.
Note - I know this is a subjective question, but I ask it to generate a reference list of most useful accessories, as opposed to identify 'the best'.
One accessory per answer, plus a little explanation of why it would be useful. I've put my top two in by way of example.
Edit: This page is meant to list accessories you've found useful, not to record basic cycling concepts (as per this thread in meta). If you would like to write an entry about a basic cycling tool, component, or concept, please use the terminology index for that.