Free Audio Conferencing / Internet Phone Software

Skype - Skype is a little program for making free calls over the internet to anyone else who also has Skype. It’s free and easy to download and use, and works with most computers. Download, register, install, plug in your headset, speakers or USB phone and start calling your friends. The calls have excellent sound quality and are highly secure with end-to-end encryption. You don’t even need to configure your firewall or router or any other networking gear. It just, you know… works. And it doesn’t just work on Windows, like some other software you may know. Skype is also for Mac OS X, Linux and PDAs using Pocket PC, with a native look and feel for each platform. Talking, sending instant messages or even file transfers work between different platforms like a charm. You can also make calls to old-fashioned phone numbers all around the world with SkypeOut program. Landlines, mobile phones... it works with almost all of them. SkypeOut is not free but it is pretty cheap. Great feature of Skype is possibility of conferencing - talking between several people at once.
Download           OS: Windows | Mac OS X | Linux | Pocket PC

ICQ - Although primary ICQ service is free text-based chatting new version of ICQ also provides Push-to-Talk function which enables you to talk with your ICQ friends over the Internet. Push-to-Talk works only between 2 users.
Download           OS: Windows | Mac | Pocket PC           

NetMeeting - Free internet conferencing software from Microsoft. NetMeeting's audio and video conferencing features let you communicate with anyone on the Internet. It also lets you conduct real-time conversations via text, with as many people as you like. Included file transfer service lets you send one or more files in the background during a NetMeeting conference. NetMeeting's Program Sharing feature lets you flexibly share multiple programs during a conference and retain greater control over the way they're used. Advanced Calling feature gives you the flexibility to send a mail message to a NetMeeting user or initiate a NetMeeting call directly from your mail address book. Remote Desktop Sharing feature lets you operate a computer from a remote location. The whiteboard function lets you collaborate in real time with others via graphic information. Windows 2000 users have NetMeeting already installed.
Download           OS: Windows           

Yahoo! Messenger - Yahoo! Messenger let you make free worldwide PC-to-PC calls. Need something to talk about? Just drag and drop pictures into your IM window to share them with your friends. It has file transfer function. You can also play in Yahoo! Games with your friend. Exchange text messages with multiple people in one IM. It has Windows version and version for mobile phones.
Download           OS: Windows

Conferencing Software Sections: Audio Communications / Internet Phones Software, Video Conferencing Software