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.