Geocron uses Google Latitude to provide locationbased cron jobs
We used several interesting technologies for the project. Because we already needed to use Google's Latitude API, we opted to handle sign up completely through Google Accounts and OAuth. For the app itself, we used the excellent lightweight Python framework called Flask. It seems to have been inspired by Sinatra, which powers a few of our APIs here at Sunlight. Last, but certainly not least, we used MongoDB as our backing data store. It was Kaitlin's first time working with Mongo, and she quickly took to it. Like I've said before, things tend to just click when working with it. [sunlightlabs.com]
You should keep clearly in mind the privacy issues, but from an engineering point of view it is a great service. The source code is avaliable on github.