I use SIP with Localphone as long as I don't need the video features of Skype, mainly because it's more flexible for me (SIP integrated in my desk phone, Ekiga and my S60 phone) and for the called party (not needing a computer)
You should generally find that we’re cheaper to most countries than Skype too, with equal or better quality.
Do you consider video to be a must-have feature? It’s something we could look into supporting since it’s all just SIP to us.
I use Socco (www.sococo.com), but more as a group-collaboration tool than calling the family. Its got a cute map-based GUI, which is OK. Its intended to keep a small group together e.g. my software development team has it running on the desktop all day. Not call-based or connection-based, you just click into somebody's home room or a conference room and voila you hear everybody there. So yes, I actually work for Sococo, but this is really a cool tool, breaks the "arranged meeting/conference call" paradigm to pieces. And its free so far.