Hm, 3 Options, of which one only supports Linux (Empathy) and one only supports Windows & Linux (Ekiga). Only Linphone seems to have platform support that comes close to Skype. So, cut it down to one.
Personally, I think Linphone looks kind of dated and their site is full of google adwords. I would hesitate to send a link out to anyone.
Linphone is "just" a SIP client. Your contacts can use it if they want and you can use another SIP client. Just like Google Talk use XMPP and people can use any client to connect to their GTalk account and then talk with anyone using Jabber/XMPP (even not Google Talk).
That's why open protocols are good. Because they're open.
I am fully aware of that. Still, this post markets them as "skype alternatives". I think the biggest problem of selling those 'open protocols' to people is that there is no consistent base client as in 'lets all switch to skype (client, protocol, server, whatever)'. I work in a company where we use all major OSes. If I came up with "hey lets switch to client A for Linux users, client B for windows users and client X for OS X", people would stone me. They want one thing: one software to rule it all.
Well, if there was a free alternative that would provide all that, at least we would be bound to RMS. (Royal Majesty Sauron?)
I would much prefer that to the secretive doings of Microskype, but you have to sell your alternatives and not underestimate network effects, especially in the network market.
"Don't use Skype! Please!" doesn't really mean much when everybody I'd (or most people) would ever want to talk to thinks that Ekiga is a funny name for a rash.
Personally, I think Linphone looks kind of dated and their site is full of google adwords. I would hesitate to send a link out to anyone.