Big Blue Button is an open source project that has developed a video conferencing system similar to the commercial offerings of Adobe Connect and Elluminate. Big Blue Button was created for the purpose of teaching online. Just as Adobe and Elluminate do, Big Blue Button allows you to present slides, chat via text, talk with presentation participants via VOIP, and share screens. Watch the demo videos here to see Big Blue Button in action.
Applications for Education
Big Blue Button is not a hosted solution so you will need to have server space dedicated to it and you'll probably need a person willing to invest some time in learning about Big Blue Button. For those reasons Big Blue Button is being marketed (although "marketed" is not really the right term for an open source project) to colleges and universities where there are more technical and human resources available as compared to K-12 public schools. That said, if you have the technical and human resources in place to implement it, Big Blue Button could represent a cost-effective way to offer online classes to middle school and high school students.