It seems like every week there is a new VOIP client out in the market with new features, cheaper calling rates, new UI. Few can really compete with Skype but a few of them have earned a place in my desktop. I currently run Skype as my primary VOIP client, and Gizmo has recently earned a place on my desktop due to their free international calling plan (and not much more - if free calling is dropped, Gizmo will quickly find its out to the trash can).
hullo Handoff - Move a hullo call to and from any phone you have in your “MyPhones” list. Now you can walk out of your home or office moving the call to your cell...never say “call you right back” again! And with hullo, if you don't have a headset…that's not a problem. Make and receive calls on any phone you choose.
hullo FindMe - Want your caller to find you or be sent to your voice mail? FindMe will hook you up. You decide who can contact you and how they do it.
Unfortunately, I haven't been able to try out hullo myself as no Mac client is available.