go go gadget wrist mounted gps!
I wish I was kidding. I just couldn't resist the Garmin Forerunner 201. It went on sale at amazon a couple years ago during christmas. It's a wrist mounted GPS device that tracks just about everything a runner could want. There's now a new version with a wireless heart rate monitor for around $100 more.
When I got it, the feature I was most excited about was what they call the "Virtual Partner". It's basically an animation of a little dude on your wrist. You program how far you're going, and how long you want it to take, and the little dude races you there. A readout tells you at a glance how far ahead or behind him you are. There are also all kinds of programmable alarms for distance, interval training, max and min speed, yadda yadda. It can whine at you for pretty much any conceivable reason you can think of, and then some.
So of course, I program the shit out of it, and for a week the damnable little thing is nearly constantly squawking at me. I'm going too fast, I'm going too slow, I'm falling behind the little dude (he's fucking relentless by the way, don't take your eyes off him). It must have looked to people watching me as if I were running on a very complicated set of prostethics, the controls to which were mounted on my wrist. The day I nearly ran into a dumpster because I was looking at my wrist, I decided I was no longer having fun. So learn from my mistake, I was using the thing without a goal, and my running was suffering because of it. Also I was unable to get into my little zone, so I wasn't getting that mental reset, which is the real reason I think I run in the first place.
So needless to say I turned all that stuff off, and now the forerunner sits passively on my wrist. It will still poke me if, for example my wife wants me home in 30 minutes, and I'll glance at it once in a while to see my average pace, or the current/elapsed time, but mostly it's a sensor that can provide a wealth of helpful information during my cool-down walk.
It also comes in handy when I'm in a new city at a conference. I'll power it on and set the location of my hotel, then if/when I get lost later on walkabout, it'll point me the way back. It's capable of geocaching as well, however setting lat-longs in it is not fun. You basically set your current location, and then edit the numbers one by one. For something attached to your wrist though, that ain't halve bad.
The garmin has a serial interface, and comes with windows software which provides graphs of speed as a function of distance, and also ones for altitude. These are cool I guess, but I'm really not that serious about anything. If you use linux or bsd there's this perl script to download the data off it. It comes off as a space seperated list of latlongs and epoc time stamps. So that plus the Haversine Forumla plus rrdtool == graphs of your runs.
Of course if you want to know more, there are a whole bunch of freaks worshiping it in various forums and user groups. You can join them here and here (you freak).