SIPPhone has recently introduced the "Gizmo Project", which they are billing as a potential Skype Killer.
As a phone client, Gizmo offers a richer set of featuers including voice mail, conferencing, and voice-mail to e-mail forwarding. As a system, it's fully SIP based and supports any SIP client or phone, as well as interconnectivity with multiple SIP-based services such as Pulver's "free world dialup" and others. It offers a fee-based service for calling to/from the PSTN.
I've installed it on my Mac and it looks pretty nice, the user interface is about as functional as Skype, but there is one missing element - NO INSTANT MESSAGING!!
The beauty of Skype is its built-in IM functionality, allowing me to first ping someone via IM before I call them, or in many cases, conduct the business I need via IM without ever having to make a voice call. Until Gizmo builds in IM functionality, I don't see it is a viable alternative to Skype.