Skype is a relatively new and extremely popular technology. It's different from other VoIP (voice-over-internet-protocol) networks as it doesn't rely on central servers, but establishes a peer-to-peer network, which can easily grow with the number of users connected. [Lots of additional information is available here: http://en.wikipedia.org/wiki/Skype.]

The software and basic membership in the Skype-network are free (see download-links in the side-bar), allowing over 100 million users free chat-, audio-, and video-calls over the internet. This includes text-based group-chat with up to one-hundred, and conference-calls among up to ten participants. Another interesting (free) feature is Skypecast: groups of up to one-hundred members can be hosted, in the form of a lecture (one person talks and the others listen), or everybody can talk (or chant) simultaneously.

Other features are available for a fee, like SkypeOut (calling normal phones from within Skype) and SkypeIn (accepting calls from outside), Skype Voicemail (storing incoming calls for later retrieval), and sending & receiving Skype SMS.

Once you join the Skype-network and are online, you can set your status to "Skype Me", which makes you visible to others who can contact you. In your profile you can tell people about yourself, your interests, and what you'd like to talk about. You can also host group-chats or Skypecasts at certain times and advertise these through the Skype-network, waiting for interested people to turn up.

[To use voice- and group-chat you need a minimum internet connection speed of 10 or 12 kB/s. Text based chat works at slower speeds, too.]