The short (and unflattering): [Gizmo](http://www.gizmoproject.com/) is an alternative to [Skype](http://www.skype.com/).
The long (and flattering): Sleek UI. Free VoIP, voicemail and conf calls. More features than Skype. Cheaper calling rate in the US. Support for SIP, so you can talk with other SIP users, not just within the closed network. And granted, Skype has a really huge network, but open networks scale better.
link: http://www.gizmoproject.com/
tags: gizmo voip sip open