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.
Rumblefish, champion of what is good and just and legally musical, will help you change your tune for the better. This company has produced a music licensing API so that you can add copyrighted songs to your YouTube videos. Of course the concept can be applied more widely and liberally. Rumblefish has even devised custom license ‘portals’ that present licensing options based on the intended use of the music. This implies that the agreement and cost of licensing a track could be more expensive depending on where and how it is used.