Keep your back side free from mud and water on those days when the weather turns nasty. The Sette Post Mount Rear Fender assembles easily with just a simple Phillips screwdriver and a 5mm allen wrench.
I bought this along with the Quest front fender for my mountain bike. I had very knobby tires that kicked up a lot of mud both on my back and my face. These have been great for muddy days. They have stopped most of the mud. They are inexpensive, and easy to install.
This is a pretty good little fender for the price. Not as nice and effective as a wraparound permanent fender, but it did keep quite a bit of rain and mud from flying up off of my commuter bike. The plastic seems pretty cheap, but hey, it's only $8.