Skype: Windows for VoIP?
One reason (I'm sure there are many others) that Windows came to dominate the PC business is that they had a published API. This led to an explosion of diverse applications - which is what most people buy PCs for, after all.
Of course, Microsoft also builds its own applications, and has integrated some of those applications into the OS platform, undercutting application developers that had contributed significantly to its success and leading to all sorts of nasty antitrust claims.
Skype has a published API. Third-party developers are able to build all sorts of diverse applications making use of a basic peer-to-peer voice over IP infrastructure provided by Skype. And while free peer-to-peer voice communications is pretty cool in and of itself - just like a windows-based operating system is pretty cool in and of itself - it's this potential explosion of applications that is the real "disruptive technology".
Questions remain. How will Skype manage the inherent conflict between providing an open platform for applications developers and providing applications on that platform - especially when they give the platform away for free and make money on the applications? Can a third-party developer bundle the Skype software with an application they sell, built on top of Skype, without violating the limitation in the Skype Distribution Terms to "non-commercial gain"? Do enough applications lend themselves to a peer-to-peer model to be worthwhile? (Interestingly, the three applications that Skype is selling - SkypeIn, SkypeOut, and Skype Voicemail - all seem to hybridize the peer-to-peer model with some level of specialized nodes, either gateways or servers.)
But the potential that is unlocked by an open API is tremendous. Tremendous enough to make Skype the Windows of the VoIP world? Time will tell.