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. (Of course, they were purchased not long ago by Microsoft, so who knows where they’ll go in the future.) 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 (essentially a knock-off of Skype)
Haven’t heard much from them lately. There are other such companies. What’s your favorite? MagicJack? Vonage? What was that one that we highlighted in Brigada a while back that MAF recommended? PC-see or something like that? Please use the comment boxes below to tell your own favorite and why.