slightly OT but the technology behind Bump was genuinely mindblowing at the time. Phones didn't have NFC or anything like that, and they didn't use much accuracy in the way of location data, so they basically just had a general "city block" location, timestamp, and accelerometer readings and would invert the accelerometer reading and look for identical accel + timestamp.

We tested it one time with like 10 phones and everyone bumping each other / the wall as a control, in the same room and it nailed every actual pairing and ignored the others. The wiki has more, but lacks the subjective experience of how magic it was.