I bought a shower timer a while ago, and it's rather brilliant actually. I highly recommend it.
It comes with a plastic bag that you need for adjusting it to your shower - great fun, sticking the showerhead inside and trying to turn on the water and the timer at the same time, and then turning off the timer again without soaking yourself... I think it would have been a lot easier with two or three people involved.
But now it works, and it has stopped me from dozing off and showering forever while waiting for the morning coffee to kick in. Instead I am juggling razors and soap and shampoo with incredible efficiency trying to beat the 35 liter beep... great way to start the day. Honest.