A legend for the following table.

Free Options: Whether or not at least some of the conference calling features are free to use.
Max Conference size: From five active callers to a thousand.  What are your needs like?
Too free cost per minute, per attendee:  The providers that allow you the option of having a dedicated Toll Free 800 number will charge you a per minute fee for each caller.
Toll Free + PC Screen Sharing:  Some providers tack on additional prices per minute.  Some add flat rates per month or don’t charge at all.

Company Free options? Max Conf. Size Toll free cost per minute, per attendee Toll free + PC Screen sharing
FreeConference Yes 150 $.10/min +$9.00/month
AccuConference No Unlim. If over 50, pls call $.079/min $.099/min
FreeConferenceCall.com Yes 96-1000 $.06/min N/A
AT&T Conference Calls No N/A $.085/min $.12/min
Skype Conference Calls Yes 5 N/A N/A (screen sharing works only with other Skype users).
WebEx Free trial period 25 $.33/min or $49/mo. $.33/min or $49/mo.

Note: free conference calling by some services are paid for using traffic pumping (see wikipedia).