ubiks

10 best uses including Tokyo:

http://www.wired.com/techbiz/it/magazine/17-03/st_best

 Manifesto of Open Disruption and Participation by Eric Paulos.

SmatMobs talks about it.

Some interesting commercial services:

Google Latitude 

loopt

Whrrl

brightkite

GyPSii

Aka-Aki

dodgeball.com

jaiku

limbo

zintin

Moximity

uLocate

citysense

MobiLuck

Plazes

WhosHere

twinkle (twitter based)

iFob

activo.jp

mapii

gumi

dokoyo

mincle

loc8r.jp

Some links: 

http://www.techcrunch.com/2008/09/28/the-state-of-location-based-social-networking-on-the-iphone/
http://www.crn.com/networking/208802663
http://www.readwriteweb.com/archives/location_based_social_networking_3_billion.php
http://www.technologyevangelist.com/2008/05/brightkite_location.html
http://blog.searchenginewatch.com/blog/080801-115059