Music and audio sharing service for artists, SoundCloud, today announced the launch of their App Gallery, “a nice home for all the apps using the SoundCloud API.” In the process they are demonstrating how startups can develop thriving application ecosystems without the “get rich quick” motivation that drives many developers to create for that other app store.

We’ve all heard it before, at events, camps, or pitching sessions – to be really successful in the consumer web you have to be a platform. But how do startups, even a funded startup with a rapidly growing user base convince a critical mass of developers to spend their precious time coding for them? In particular, why does a handful of Swedish guys who moved their company to Berlin just because the parties are better, think they can build a dynamic App Gallery?

The answer, which is both very simple and very complex is this: they have an insanely great service that provides a very high value to their users. “Punkt slut,” as we say in Sweden. Because SoundCloud’s service helps musicians and DJs get their music heard, there is an intense interest in developing applications around it. The best example of this is CitySounds, a SoundCloud app that has tracks from artists in 32 cities around the world. The developers are motivated to get the most out of the service in order to get the music “out there.” Not because they think a few thousand bored commuters will pay $2.99 for another finger tapping game.

The natural order of business has been that big businesses work with big businesses and small businesses work with small businesses. The iPhone app store is an exception that proves the rule. When big companies work with small companies and the power relationship keeps the startup at the mercy of the gatekeeper, bad things can happen. With the SoundCloud App Gallery, developers who think the SoundCloud service can help them can work with an equal partner. It’s even in the developers manifesto, “We recognize that only through your apps and hacks, can SoundCloud fully realize its potential.”

The symbiotic nature of the SoundCloud App Gallery will ensure its success, because both sides need the other to grow and improve. SoundCloud’s AppGallery would be one for the textbooks, if they, you know, actually made textbooks about how to become a platform with an eager community of developers.