Big Blue Button (often written as 'BBB') is an open source (like Moodle) web-conference system. Although it is free and open source, it provides similar functionality to systems costing many thousands of dollars such as Adobe Connect, Cisco WebEx, and WiZiQ.
This plugin allows Teachers to add BBB conference rooms to courses in just the same way as they might add a Forum or Quiz ! There's almost no learning curve to using the plugin - although of course Teachers, and Students, do need to know how to use BBB once they add it to a course
BBB is used in MoodleCloud, for the iMoot, in MoodleBites, and many other examples - and its growing list of features (including a mobile client in development) make it an excellent choice to use alongside Moodle.