As I'm based in London, I started off using the Run London routeplanner (www.runlondon.com) which was pretty accurate but didn't always take account of the distance added by winding roads.
I now use Nike +iPod, which is extremely accurate, moreso if you calibrate it properly. I also have a Fitbug pedometer (www.fitbug.co.uk) which is extremely accurate - don't slate pedometers until you've tried this!) and also gives me discounts on my health insurance premiums through a special scheme with my provider, PruHealth.