Skype is that free voice and video site purchased by eBay several years ago. It is my preferred way to have a live discussion while sitting at my computer. With or without the video, the audio is fine when you have a good computer, broadband, and a quiet office. Even if the room is noisy and somewhat chaotic (like a classroom of antsy students), the audio with video is powerful. Given most school libraries have wireless Internet, thanks to annual Federal E-Rate funding, Skype is an ideal way to bring authors, scientists, college admissions officers, and other guests to the schools.
Skype's special site for teachers is worth checking out, even if it is mis-named. Skype in the Classroom should be "Skype in the School Library".