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.
2010 is not yet half over and already Music Hack Day is hitting its third city of the year: San Francisco. On May 15th and 16th, scores of hackers will convene at the Automatic Lounge for a 48 hour hack fest themed around all things music.
The Twitter Streaming API has been successfully used by Yahoo!, Google, and Microsoft, who have been given access to the full feed of all public tweets. Twitter has now enabled this "firehouse" of data for a number of other companies.