Ribbit, the web phone company recently acquired by BT for $105 million, has opened its developer platform to the general public. Although the Mountain View, Calif. company makes useful applications like the recently-updated Ribbit for Salesforce, the most exciting part of its technology is the platform, which promises to allow anyone to add phone capabilities to their web applications.

The platform has been in beta testing for the last few months, and there are already more than 7,500 members signed up in Ribbit’s developer community. Until now, if anyone wanted to voice-enable their applications with Ribbit, they needed the company’s approval, which meant that there was a waiting list. With today’s announcement, Executive Vice President Crick Waters says Ribbit is now closer to Google Maps, which allows anyone who wants to use its application programming interfaces (APIs) to add mapping to their web apps.

Ribbit also plans to boost those developers numbers, and to encourage cool uses of its APIs, by offering a $100,000 prize for the “killer app” built with its platform.

Broadly speaking, there seem to be four markets that can make good use of the Ribbit platform, Waters says: business productivity, media/marketing such as design agencies, social communications such as SayHear.org and a new category that Ribbit is announcing today, carriers who use Ribbit’s “bring your own network” model. That last part is a direct outgrowth of the BT acquisition. Ribbit normally provides the network infrastructure, but in BT’s case that wasn’t necessary — the British telecommunications giant wanted to offer Ribbit features across its own network. It looks like BT isn’t being too protective about this, since Ribbit now offers that ability to other telecom networks who, like BT, want to add Ribbit to their offerings.

Although Ribbit dubs itself “Silicon Valley’s first phone company,” others offer similar web phone platforms — Ribbit executives named Ifbyphone and Jaduka, and we should also add IntelePeer, which is used to add voice capabilities to big-name sites and services like WebEx and Facebook. But the competition subscribes to a more closed, professional model, compared to the Ribbit platforms new openness (which I described above).