What's your most useful cycle accessory? 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.
Fenders/Mudguards - front and back.  I ride all through winter and these help keep me dry. There's usually only one or two dozen times a year that I'm really riding in the rain for my morning or afternoon commute, but there's a good 3 or 4 months where rain comes through often enough that I'm riding through a lot of puddles.The rear fender prevents the classic skunk stripe, while the front fender primarily prevents splashing onto my feet and ankles.

These also help keep your bike clean and should help increase the lifetime of your chain/gears (or at least give you an extra few days before it's time to clean and lubricate everything when riding in winter).

I don't remember the exact model, but they're basically these fenders from Planet Bike that include a mud flap.