I recently encountered the annual Worldwide Photo Walk and it all starts this Saturday (July 18, 2009). It is a free event where volunteers lead a bunch of people (limited to 50 per city) on a predetermined route which lasts around two hours. Although it is indeed a free event, there are some very worthwhile prizes waiting for a few lucky winners!
It would have been fun attending the one being held right here in Stockholm, but being late to the party, all spots are taken ages ago.
There is an early bird walk being held here too where there still are a few spots left, but since it starts as early as 04:00, I am guessing that it will not be filled up quite as fast.