If there're things that stick me to Windows, one of them would be Google Talk (GTalk). I use it as my primary instant messenger as well as a PC-to-PC phone to call my home in Thailand.
Why don't I use Skype?
Firstly,I feel that GTalk provides better both quality and integrity of voice call than Skype (it's subjective; my feeling). Secondly, its interface is quite simple and minimal, compared to Yahoo or Skype, so that my family members can familiar with it quickly. Their computing skills are below average, so letting them face something that is confusing is not a good idea.
For instant messaging, I use Kopete, a multiprotocol messenger for KDE, but it cannot be used as a voice call client. So I have to google around and found Tapioca Project. It uses Jabber protocol plus libjingle, released by google, to make voice calls with GTalk. In fact, Kopete has an experimental feature that can make voice calls to GTalk using libjingle as well but it doesn't work for me. Also, it can crash Kopete itself when using voice call.
Tapioca comes with GTK interface, tapiocaui, which is very simple, too simple! It has only login, contact and chat windows (with voice call function). There are no configuration options, account management and avatar support. However, if you don't like tapiocaui, there is an alternative UI for Tapioca, named Landell. It is written in C# and requires Mono to run. It has far more flexible user interface than tapiocaui plus account manager and avatar support. However, it can't be embedded to the system tray and has some problems with Thai language. I couldn't send Thai message via Landell.
Tapioca with tapiocaui can flawlessly make voice calls to GTalk client. However, it has no incoming call and message alert popup and sound, just a blinking icon in the system tray, so that I missed some calls and chat. I tried to build the latest version of tapiocaui (0.3.9.1) but was not successful.
Tapioca and libjingle are not new. They have been released for more than 6 months. I had tried them before on PCLinuxOS. But at that time, there were no packages for PCLinuxOS so that I had to build them myself. Tapioca and libjingle depend on many packages such as ortp, farsight, dbus, dbus-glib and many more. I had to build them all from source and, BOOM!, they screwed up my box. After that, I switched to Kubuntu -_-"...
For you who are using Ubuntu family, there are Tapioca and Landell packages for Breezy and Dapper. Just add "deb http://extindt01.indt.org/VoIP/apt dapper main" for Dapper and "deb http://extindt01.indt.org/tapioca/apt breezy main" for Breezy to sources.list and then apt-get.
I wanna say that Tapioca is the best answer for ones who need GTalk voice call on Linux, at this time, though. It does its job excellently but its UI is too minimalist. Improvements to UI will make it more attractive.