This paper demonstrates how useful content can be generated as a by-product of an enjoyable mobile multiplayer game. In EyeSpy, players tag geographic locations with photos or text. By locating the places in which other players' tags were created and 'confirming' them, players earn points for themselves and verify the tags' locations. As a side effect of game-play, EyeSpy produces a collection of recognisable and findable geographic details, in the form of photographs and text tags, that can be repurposed to support navigation tasks. Two user trials of the game successfully produced an archive of geo-located photographs and tags, and in a follow-up experiment we compared performance in a navigation task using photographs from the game, with geo-referenced photos collected from the Flickr website. Our experiences with EyeSpy support reflection upon the design challenges presented by 'human computation' and the production of usable by-products through mobile game-play.