While many people use Skype for its free voice over IP (VoIP) services, Linux users have a love/hate relationship with it. Yes, Skype will run on some versions of Linux, but it doesn't run on all of them, and the Linux version (2.2-beta) lags far behind the Windows version (Skype 5.3). That's three major generations behind. Need I say more?
Much as I dislike Microsoft's recent purchase of Skype and even though I think Skype's technology is held together by bailing wire and duct tape, maybe Skype will become better for Linux with Microsoft. After all, it couldn't be much worse!
That said, there are numerous Linux VoIP programs and they're also free as in "free beer," as well as free as in "free software." Most of these programs use the open SIP (Session Initiation Protocol) standard or Extensible Messaging and Presence Protocol (XMPP) If they use the same protocol, you should be able to use one client to call another. To do this, of course, they must be on the same SIP or XMPP network. For example, I use the Ekiga.net VoIP network for SIP calls.
None of them, however, can work natively with Skype. Skype is a proprietary system and locks users into using it. You can set Skype up with Skype Connect to work with SIP VoIP systems, but this is a paid service. There are efforts afoot, such as the karaka Google Project to create Skype/XMPP gateways, but they're for developers, not ordinary users.
There are also numerous Linux based IM clients, such as Pidgin, that also include some VoIP support.
The Linux VoIP clients also have some other things in common. On the down side, none of them allow desktop sharing. That's not a feature I use very often, but it's one that's used all the time in technical support operations. In addition, except for the Google family of voice applications, none of them make calling from VoIP clients to phone company telephones easy.
So, that said, let's take a brief survey of Linux's Skype alternatives.
Linux-friendly VoIP clients
Ekiga: This program is perhaps the best known of the Linux VoIP clients. Formerly known as GnomeMeeting, it's an open source VoIP and video conferencing application for GNOME. Besides SIP, Ekiga also supports the H.323 videoconferencing protocol. With this protocol, Ekiga supports Microsoft NetMeeting interoperability. It does not, however, support Microsoft's newer Windows Meeting Space replacement for NetMeeting.
In my experience, Ekiga works quite well. In particular I was also pleasantly surprised at how well it worked with NetMeeting. If it weren't for the overall value in the Google Chat/Talk/Voice package, Ekiga would be my favorite Linux VoIP program. That said, if free software is a priority for you, then Ekiga is the program for you.
GNU Telephony is a project that has just reached the 1.0 stage. With the release of the GNU SIP Witch 1.0, a SIP server, this program is much closer to being something that end users would want to use. For now, though, GNU Telephony is better left for free software developers rather than users.
Google Chat / Google Talk / Google Voice. Sorry if I'm confusing you bu using three different names, but Google has made describing its VoIP service more than a little confusing. I'm reminded of the story of the blind men and the elephant. Depending on what part of the elephant you're touching, you'll have an entirely different perception of the whole beast.
Here's the truth of the matter: If you have Google Chat, which is Google's IM service, and Google Talk, which is its VoIP and video service, and Google Voice, which is Google's private branch exchange (PBX) service, you end up with the full range of VoIP and video services, including the ability to call landline and mobile numbers.
The only downside is that there isn't a client for this. Instead, Linux and Mac users need to install a Google Talk video and voice plug-in to their Web browsers. Only Windows users get a client at this time. On the plus side, you can use it right from your Google or Gmail page and it works quite well. To me, regardless of operating system, this is the real Skype replacement.
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.