The one we all use is, of course,
It’s essentially free, good quality, easy to grasp. You can put money on account then call into the local grid. And they continue to innovate. You can pay a premium fee ($15 per month?) for conference-calling over the Internet (up to 10 or 15 people).
But there are other solutions. We have recommended, in the past
http://www.oovoo.com (Think Skype with a bit of an attitude)
Haven’t heard much from them lately. There are other such companies, like Vsee, highly recommended by MAF because of its strong injury-list.