There is now a wide range of VoIP software on the market. The most common software based package for end users being Skype. But, there are a lot of alternatives. Many of the instant messaging platforms from the early days (AOL etc) and more recently GoogleChat have started introducing VoIP elements and features to their basic software application and from the usage statistics, people are taking note and using them!
It doesn’t stop there, Apple have also introduced VoIP capabilities to their iChat application along with many new versions of the up and coming Gizmo5 application and Empathy is another popular GPL application.
There is now also a good range of server side software in the VoIP sector. The most well known of this bunch is the Asterisk system which is Linux based and has been in development since around 1999. Asterisk does however have some competition in the form of GNU Gatekeeper (another popular development from the open source community) and FreeSWITCH have recently updated their offering also.
I expect over the coming years, all of the aforementioned systems will gain competition as usage of the VoIP protocol really takes off and hopefully saves people a bit of money in the process.