Here is a list of free conference companies in the united states. Some even support VOIP!

http://usemybridge.com Free conference bridges. No signup. No limits. No cost - ever. UseMyBridge donates a portion of every minute to a charity of your choosing.
Free Conference Video Conference company supporting VOIP and video (in the future) built on Asterisk
No Cost Conference No Cost Conference
For a conference call check out the services of Free Conference Calling.
Free Conferencing Corporation Free Phone Conferencing.