Desperately Seeking Serendipity is an must-read article on cities, serendipity, urban planning and design, and the process of discovery in general. Aside from some minor quibbles with his grouping terms and terminology, the article is excellent.
This part really stood out as a feature that pretty much every mapping and discovery service needs to implement:
"The map I want is the one that lets me shuffle not just through my friends’ preferences but through annotations from different groups: first time visitors to the city; long-time Vancouverites; foodies; visitors from Japan, Korea or China."
Opir recently got a mention on the Maxwell-ALIVE game artist blog.