I'd vaguely heard of this, maybe somewhere in the Skype forum, and didn't really know what it was
It's a client program to use wi-fi or 3g data access for VoIP applications in suitable phones - mostly Symbian Nokias
Apparently the latest version yesterday is also usable with SIP accounts
http://www.fring.com/forums/showthread.php?t=952
I don't yet have such a phone, so I haven't studied their website in much detail so far.
Of course there are other client programs, but perhaps most of these are dedicated to the provider that developed them