What's your most useful cycle accessory?
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.