There are a number of ways to discuss API popularity. Our metrics are based on developer engagement. The "track" functionality on ProgrammableWeb lets developers declare an interest in receiving updates on particular APIs. Which APIs and categories are seeing the most activity.
Gnomedex, the annual tech conference in Seattle hosted by Chris Pirillo, and Microsoft Live Search, are teaming up to sponsor a developer's contest dubbed "Will Code for Green" where judges will accept mashup entries in two categories: Economy and Ecology.