Voice-over-IP is fast becoming one of the most important tool for the internet-savvy. Whilst there are several ways of doing this, afaik, there are only two technologies that are worth looking at, i.e., the SIP-standards based services and Skype, a P2P technology.
I will not go into the "issues" of using Skype but I must confess that I do use it alongside my SIP-based GizmoProject account. I like the way Skype optimizes your internet connection – heck, I can use it even when connected to the internet using 3G. :)
Last week, version 1.4 beta was released for linux. Whilst this is not as feature-rich as its Windows or Mac OS X equivalent, it is worth the download.