The NYTimes.com has a good rundown on Smartphone Training Apps and other personal technology that can help you get fit. One of my favourite is this:
The Nike + iPod Sport Kit is made especially for runners (it can be used in a gym for cardio workouts as well); it tracks time, distance, pace and estimates calories burned. It uses a $29 sensor compatible with a Nike+ shoe and beams information to an iPod or iPhone. It does take some effort to calibrate. You will need to run a known course to set it up for highest accuracy.
I take issue with the part I underlined. It’s pretty easy to calibrate. I took mine to a track once that I knew was a quarter mile and calibrated it easily. As it was, it was pretty accurate to begin with.
If you can’t find a track, consider using maps.google.com to map out a quarter, half or full mile (or kilometer) and then calibrate it there. (try to find a flat stretch).
Another tool I use is my Polar heart monitor. Polar makes many different kinds: I use a basic one to allow me to train at a rate that’s not too easy, not too hard.
Finally, a good ol’ running watch / stopwatch is always handy, even if it isn’t digital.