That is, if you're in the U.S. All too many of the Google combination's features don't work outside the U.S. You can still call out of the States, for example to someone using an XMPP service in the U.K., but they can't use the Google package to return your call. Hopefully, Google will launch its full range of services worldwide, or at least in the rest of North America and Western Europe, sometime soon.
Jitsi, formerly SIP Communicator, is perhaps the most full-featured of the Linux VoIP clients. It supports SIP, XMPP, and to one degree or another, VoIP to AIM, Windows Live, Yahoo!, and others. It also runs on Windows and Mac and there's an Android port coming.
With all that I can't give Jitsi my full approval. In my experience, after running it on Ubuntu, Mint and openSUSE, it never works that fast. That might be in part because it's a Java application. It also doesn't work with Ekiga.net due to some protocol problems. It will work, however, with other SIP networks such as Iptel and ippi.
For me, it's too much trouble for not enough benefit. But, they are actively working on improving it, and unlike Google's offerings it's not a U.S.-only package, so I plan on keeping an eye on it.
Linphone is the one product that came out of the late Linspire Linux that's still going. While it's "only" a SIP-based softphone, it has the advantage of working not only on Linux and Macs and Windows PCs as well, but of coming in versions that work on Android, iPhone and BlackBerry. As such, it is, to the best of my knowledge, the single open source VoIP client with the broadest architecture support.
In my experience, Linphone works OK, but it doesn't knock my socks off. Jitsi has more promise and Google has more real-world functionality.
Besides these there are many other Linux VoIP programs. These are just the most well-known ones that I've used.
If I were looking for a Skype replacement today, I'd have to go with the Google package, but then I live in the U.S. and I would really like it if there was a single, easy-to-use and international Google Talk/Voice combination. Over the years, I've liked Ekiga, but of the standalone clients I think Jitsi has the most potential to be a great VoIP client.
As always with open source software, you don't need to take my word for it. All these programs are free to download and use. The SIP and XMPP networks are also free to use. So, try them out for yourself and see which works best for you and your buddies. One way or the other, you'll find one that's better than Skype.