Free as in Speech-ness:
Skype: Proprietary software, proprietary protocol. Also owned by Ebay. Booo.
Gizmo: Proprietary software, open (SIP) protocol. Not bad.
Wengo: Open source software, open protocol. Vive la France!
Free as in Beer-ness:
Skype: Skype has been offering free calls within the US and Canada, though that runs out at the end of the year. I think that that deal only applies to domestic calls, but I can't find an international rate *to* the US.
Gizmo: Gizmo has just started giving free calling between "active" (a few calls a week) users in sixty countries, including the US and Denmark. All you have to do is put your landline/mobile numbers in your profile (and have your friends do the same) and those numbers are free! It isn't clear when/if this deal runs out.
Wengo: If you buy 10â‚¬ (about $12) worth of credit, you get free landline calls to "Star Countries" (which include the US and Europe). The deal apparently ends in September, though.
All of these have free computer-to-computer calling, of course. Just have to get all my folks on the same network...
Skype: The rate to Denmark is $0.021/minute. I can't find the international US rate.
Gizmo: Calls to the US are otherwise $0.01/minute. Calls to Denmark are $0.017/minute.
Wengo: Calls to the US are 0.008â‚¬ ($0.01). Calls to Denmark are 0.011â‚¬ ($0.013), the cheapest standard rate I've found so far.
Skype: Not bad, not spectacular. Cute sound effects.
Gizmo: Plenty of bells and whistles, including easy, built-in call recording and a nice little feature called "Map It." Has an Adium plugin.
Wengo: Not just a VoIP client--also does instant messaging (AIM, Jabber, etc), text messaging, and video conferencing. Has a Firefox plugin.
My dad constantly raves about Skype's sound quality--his company worked with Skype's codex before VoIP got off the ground. I really haven't noticed any difference between it and Gizmo, though. Perhaps the difference will be more apparent when I go overseas? I just downloaded Wengo, so I haven't tested it much yet. If it's extraordinarily good or bad, I'll say so later.
I guess this post is just getting all this information sorted out in my head. I haven't decided which client to settle on for my trip abroad. Have any opinions?