Smule have a lot to answer for.
In odd moments on train journeys, waiting for meetings or when bored of television, I'm finding myself turning to one of their apps with increasing frequency. I first came across these guys when someone played the Ocarina video at Future of Mobile last year, and I've downloaded a few of their apps since then: the Ocarina itself, SonicLighter, and most recently Zephyr.
I suppose the best way of describing these would be social, geographic toys: do something, and see other people round the world doing similar things. In the case of SonicLighter, all you're doing is running the app; there's a screen when it begins where you flick-start a virtual cigarette lighter, but all the action occurs on a world-map where you can see locations from which other SonicLighter owners have used the app in the last 24 hours.
Ocarina and Zephyr take this principle and make it more expressive - in the case of the former, you play a song on your iPhone or hear other songs being played around the world (together with a lovely visual representation of where they're coming from, on a nicely drawn globe). Zephyr lets you trace out a message on the screen of your iPhone, send it off on its travels around the world, and brings other messages to you.
It's difficult to overemphasise how cute I find this stuff, and how much I get simply from being aware that there are other people out there, anonymous to me except for their location and the fact that they're doing the same thing as me: the connection is the meat here, and the activity around it just gravy. There's something wonderful and very human about seeing a message slowly get etched out onto your screen, after watching it ebb its way across the Atlantic Ocean, or bounce from city to city.
And of course, Smule have done a great job of keeping the whole experience smooth: no registration required, good maps, and a carefree exploration of the apps seems to be encouraged throughout.
I also note that they're starting to do a bit more with them. SonicLighter now asks me "who I'm lighting up for" and gives a number of options. Here's a distribution of who lit up for who in the recent US election:
Lessons learned here? The connection can be the application. The trivial can become interesting when you can see others doing it at a distance (perhaps only then). Identity isn't a necessity for social software - I don't know or care who the other people are on these apps, that's half the fun. And location-based apps can be delightful too :)