We are currently looking at setting up a VOIP PBX to allow IP dial-in conference calls. One solution looks to be Asterisk--An open source PBX offering just about every feature you will find in commercial solutions. Asterisk@home is a project which aims to make a basic install easier, this project provides a CD which will deploy a fully functional Asterisk PBX with the underlying OS and a fully operational web interface to ease configuration and administration.
You may wonder why go to this effort rather than using a simple program like Skype? Unfortunately its a case of compliance, followed by securityAdd to this the fact that call quality with Skype degrades quickly when there are more than three callers, its not much good for commercial use.