I've been using Skype since their first beta release. Anyone involved in instructing (or communicating for that matter) online should use this tool. It's excellent (if you're not familiar with it, it's basically a tool that allows you to make phone calls online (for free if the other user has a skype account as well)).
S & B has a nice overview of Skype:
"One of Skype’s most high-powered adherents is former Federal Communications Commission Chairman Michael Powell, who said in January 2004 at a telecommunications conference at the University of California–San Diego: “I knew [the traditional telephone system] was over when I downloaded Skype…. The world will change now inevitably.”"