Runkeeper
I don't have an iPhone, but Liz does, which means that much of the time I do too. Although she has installed a game or two for me, I have to say that my all time favorite app for it right now is Runkeeper (free version). Obviously we've really gotten back into running lately, having fully bought in to the barefoot running school of thought. Runkeeper gives you a lot of information about your running. You simply turn it on and start it at the beginning of your run, and shut it down at the end of your run. You can even pause it during your run if you stop for any reason, such as to take a phone call.
Runkeeper is GPS based, so in addition to timing your run, it can also tell you how far you've traveled. It will plot your path onto google maps, tell you how many minutes per mile you're averaging, as well as the total calories you've burned during your run. Slicker than that, it uploads each recorded run to their website, where you can see charts and graphs of your aggregate data.
We haven't yet subscribed to the full reporting utility ($20 year). We probably will, ultimately. We're saving it as a motivational gift for ourselves whenever we need a little more encouragement to keep running.
Speaking of motivational gifts, we have promised ourselves we'd use some of our tax return money so that each of us can get a pair of Vibram Bikilas when they come out in April or May. However, we've challenged ourselves to have to do a 5 mile run in order to earn them. We've also challenged ourselves to run a few of the 5k events in East Texas this year, and we're going to try to do a half marathon at some point in 2010. We'll have to do a little searching to find one that we can participate in. So far, I've ran 1/3 of a 1/2 marathon, so I'm feeling like it's a fairly realistic goal. I probably won't be breaking any records, but as this will be my first event larger than a 5k, I'll be happy to complete it in any reasonable time.