This is a small (matchbox-sized) unit with a single button on it (on/off), and it basically does one thing – logs location data at a regular (user-specifiable) frequency.
Why would you want to do this? Well, apart from tracking cats or taking on board the subliminal message that you might want to follow your unfaithful husband or wife (the unit is called – rather nastily – “i-gotU”), one of the main reasons is this: you can use it to geotag photos automagically.
The nice thing about the unit is that you can use it with any camera, not just your fairly-crappy iPhone (already GPS-enabled) one. How? it’s really, really simple: you just sync the clock on the GPS unit to the clock on whichever camera you’re going to use. Bingo. The supplied software does the hard work of matching geo-location to time and then (here’s the cool bit), writes that data back as EXIF to the image. In human speak, that means when you upload your pictures to Flickr or import to Picasa, the location of the image is automatically logged. And that, ladies and gentlefolks, is cool.
None of this is rocket science (although as a kid I would have probably wet myself in excitement about a matchbox-sized Bond-style tracking unit..) but the cheapness, small size, simplicity with which this particular idea has been executed is pretty compelling.
Simplicity, as always, is a fine thing.