Being an avid internet gamer, I can confirm that there are several VOIP applications in use by this community, to provide a functionality which is very expensive to achieve on a standard phone network.
The Two Main ones are
Teamspeak
TeamSpeak - Welcome to TeamSpeak
and skype
Skype official website - download Skype free now for free calls and internet calls
They provide the users with the opportunity to have groups of people all involved in the same conversation, and as such also require a degree of etiquette in their use so they don't become unusable (with everyone talking at the same time).
Both systems have free and charged options, but a simple server to provide a group chat functionality for 5 or so people is reasonably simple to set up (with occasional problems with firewalls, but they both have very active user communities who have numerous forums for assistance on such matters).
I have personally also used them at Xmas and big family occasions to enable people from different parts of the world to all communicate together and be involved.
A simpler system if you just wish to talk to a single friend over the net is good old MSN Messenger, which so long as you have a user account and a microphone will enable you to have a VOIP conversation with another MSN Messenger user.
I would after much experience with these technology recommend the use of a proper headset, preferably one of the USB ones with their own sound processing technology, these are very useful as they mean that you can continue to use your computer normally and have all the VOIP conversations managed by a separate sound processor. This removes the problems of sound balancing as you have physically separated the two sound channels so all you will hear in your head set is the VOIP conversation and by using a headset you should also ensure that those who you are talking to only hear your voice and not the other sounds from the computer.