Skype and Ekiga are the native linux answer. If you install Wine, you can also use Ventrilo. It works, but depending on your hardware/system performance, it could be somewhat choppy.
Model: HP Pavillion dv9700
CPU: Intel(R) Core(TM)2 Duo CPU @ 2.59GHz
Videocard:Nvidia GeForce 8600M
Wifi:Intel PRO/Wireless 4965 AGN Network Connection (rev 61)
Display: 1680 x 1050
OS:Fedora 11 x86_64 Leonidas