Powerstart Victory lap timer.
www.powerstartracing.com
I got one of these about a year ago, and I must say it was BY FAR the single most helpful item to increase my race speed I've ever had. I love it, and it only comes off my bike for washing.
It straps to your front fender, just in front of the number plate, and has a trackside infrared unit that trips the timer each time you go by. It will store all your laps, tell you at each lap's end whether you were faster than the lap before by using green and red lights, and can be used with 2 infrared units to time sections, like trying inside or outside of a corner. Mine has worked flawlessly, and has been in BIG crashes and soaked down by sprinklers at the track. It actually makes going to practice by yourself fun, and I'm always trying to outdo myself, getting better all the time while having fun.
Many of the big schools use them (Gary Bailey, Donnie Hansen, etc.), and I know why. I hope noone I race against ever gets one. :)
2002 CRF450R
2001 CR250R