Used to use the nag me, but I have since switched to the ATR Tracker service. Allows much better control of your ‘stable’, where you can put a sentence or two beside each horse, which is very useful if you track numerous horses and cannot remember why you tracked it in the first place. You can also set the number of runs before the horse is removed from the ‘stable’, which I find useful so I am not being constantly e-mailed.
One advantage nag me does have is it e-mails you the day before rather than over night as ATR does, though this is of little inconvenience.
Both services do basically the same thing, just a matter of personal preference (both are free)