I have and use everything everyone has mentioned so far!
1) I use mapmyrun to get a general sense of mileage to plot out a general route. You can miss corners and not get direct lines if you plot the run from too far out on the satellite, but it will do the job for figuring out distance.
2) I first started with the Nike+. It does suffer from inaccuracy as you can only calibrate it for 2 paces (running and walking). As you vary from that pace, the inaccuracies start to creep in. My first sensor was fairly accurate with no calibration (to within 0.1 miles over 5 miles). That died while it was still under warranty and my second and current sensor is much more inaccurate right out of the box (to 0.2 miles over 3 miles). I think it is a good sort of beginner tool for general purposes as long as you realize (a) you are not running the distance it is telling you and (b) therefore your pace is incorrect. Also if you already have a nano, it is a fairly cheap investment to get it going. I think it is $29 for the sensor/transmitter pack. You don't need the special Nike shoes, you can buy a pouch to tie the transmitter (the accelerometer) to your laces (Marware is the brand I have, see
Amazon.com)
3) I have a Garmin 305 that I bought in Nov. of last year. That is great. It claims accuracy to within 10 meters. I don't know if I buy that, but ok. It's a nifty toy and you can set up all sorts of different workouts and have a virtual training partner, make it alarm on certain conditions, etc. It's pricey, but cheaper than it used to be given the introduction of the Garmin 405.
Really, all 3 options are fine. It's just a matter of how much you want to spend - free, a little bit, a lot!