BuddyPing is one of those rare digital services which you look at and think "that's so useful I'd always assumed it must be impossible or someone would have already done it by now". What's it do? It tells you where your friends are by locating their mobile phones. It tells them where you are. See the BuddyPing widget over on the bottom left of my site? Click on that and it tells you I'm at work. Click on it later and it'll tell you where I'm spending my evening (in Greenwich, eating sushi, since you ask. Privacy is so web1.0.)
London's a busy place. The number of times I've been a few feet away from close friends and only found out about it later are ridiculous - the number of times it must have happened unbeknown to any of us I can't imagine. So there's a mobile version of the site which pretty much solves that problem. Superb.
TechCrunch has a detailed and excellent review here noting the relevant competitors (notably of course Google's Dodgeball, which if it ever manages to integrate with Orkut (ReadWriteWeb) as planned will nicely sew up the Brazilian market and be of no significance anywhere else) and Ivan at Snipperoo seems quite the fan too. And the tech support? I - brilliantly, I'm sure you'll agree - entered the wrong mobile phone network when I signed up and they had it fixed in a trice. It's all good.
Update: by an entertaining coincidence, Steve Yelvington writes that Boost Mobile (one of the world's very few successful MVNOs) is launching a service in the US with that very friend-finding feature. He points out that it "needs an off-switch". And thus I return to Michael Bywater's "having something to hide is not
contingent on guilt but on autonomy". Indeed, it seems likely that an off-switch is going to be a necessary feature of these services. I've occasionally thought it'd be nice to be able to turn MyBlogLog off (without the cookie-deletion hassle) so as to snoop through the web unobserved, and Fred Wilson tells us the MyBlogLog guys have implemented an elegant solution for that need. inevitably people will experience the same need to a far greater extent as they become trackable corporeally.