Serious!!!
Once again: it syncs with the iPhone address book.
I use it for reverse look up (phone number to address book entry) together with my telephone system in my home office, and that’s build around an AVM “FRITZ!Box“. The glue software got implemented by myself in ruby (w/o Rails).
Why ruby? I thought, I could make it run with Cocoa Ruby on the iPhone. But you know yourself: Apple fights those kinds of things. And just for running it with a GUI on a usual Mac OS X? No, that’s not worth my effort.
Update / 2010-06-16 11:45:
I forgot mentioning xing.com here. Downloading vCards from xing and adding them here – that’s just awesome!