Just found about about this and it sounds quite fun! People hide 'treasure' (normally things like small toys, coins, keyrings etc.) places and then log the co-ordinates on a website, you put the co-ordinates  into your satnav and then go hunting. When you find it you can take some of the treasure as long as you have something to put back in the cache in exchange and there is a log book you can fill in.
There is a cache less than a mile from my house, it must be well hidden because I go there loads, I'm going to investigate today or tomorrow with my GPS to see if I can find it and swap some treasure!
Has anyone here had a go of this before?

Heard of similar some years ago when a daily or local paper 'hid' a car & someone thought it was on our property. Haven't heard about the on-line game but have real life searching every day, i.e "where's my keys/gloves/hat/coat/belt/credit cards................." . No reward at the end though!

http://www.champdogsforum.co.uk/board/topic/111448.html

there were quite a few posts on above link with more info

I used to own a pub and one of my regulars organised this sort of thing. He hid kinder egg pods with a number inside them so participents could note the number and log their times etc online.

Always fancied doing it but was busy cooking lunches for them hungry hunters return!