Susan,
If you can afford it (at least £120 - RRP £220), the Timex Speed Distance monitor is very good.
Two or three threads on this in the forum.
It gives pretty accurate measurments of distance, speed, pace etc by using GPS (track your movement by satallite, believe it or not and transfers data to a watch).
Supposed to be 98% accurate. I've had it for 3 months and love it.
But I guess you've got to be pretty keen to spend that sort of money.
cheers