Conference calls (also known as voice bridges) can be incredibly handy, whether for having a business meeting with far-flung participants or just chatting with all of your siblings at once. My local phone company lets you set up a three-way conference, but that's often not enough people. A service called Free Conference lets you set up a conference call with up to 100 participants, and is, as the name implies, free.
There are two types of free conferences: Web-scheduled conferences have a guaranteed time slot at a specific time for a specific number of participants. You reserve this time slot in advance using the Free Conference web site and call the dial-in telephone number the site issues to you. Reservationless conferences allow you to call any time you wish, and you can choose to call any of the conference bridge phone numbers (a list of them is here) but there is no guarantee that there will be sufficient space for your conference on any given conference server at any given time. Reservationless conferences are cool because they're anonymous, you don't even need to create an account at FreeConference.com.
The free conferences include spiffy features such as muting (where the "presenter" can keep listeners from interrupting) and music-on-hold while the first caller waits for other participants to call in. You can upgrade to calls with extra features — such as having a toll-free number for participants to call and conference recording — but that's optional. Several of my colleagues have used the service (both as a conference originator and a participant) and swear that it's a great service, even at the free level.
Try it for yourself at FreeConference.com