Fring, a successful mobile application that merges an instant-messenger with the ability to place calls over the internet, has recently opened its application programming interface (API) to other developers.

The new API will allow developers to build applications that deploy Fring’s interface, instant messenger, file transfer and, perhaps most significantly, its “presence” awareness capability. Fring taps into MSN Messenger, ICQ, Skype and Google Talk. Like any IM service, it lets other users know when you are available online. Together, presence and location awareness could in theory combine to create some interesting applications.

Fring’s API currently only works with Nokia’s S60, v. 9.2 phones and the company has not developed any of its own spin-off apps to show off the API’s capabilities. However, the company says it’s only a matter of time before it becomes available across multiple platforms and hundreds of devices worldwide, a reach that may make even eager iPhone developers drool. Fring actually does have a version of its app for hacked iPhones, which, as Read/Write Web’s Marshall Kirkpatrick reports, is getting over 100,000 monthly downloads globally.

The Israeli company raised $12M last year.

[Check out MobileBeat2008, VentureBeat’s mobile conference on July 24. Vote for your favorite mobile application or service company.]