SKYPE - Skype is a great way to bring guest speakers to your classroom without the cost of transportation or missed work time.
ooVoo Video Chat - free chat with up to 12 people
Veeting Rooms - Unlimited free videoconferencing for sessions of up to 30 mins. Max 5 people for video, 10 for audio. Integrates presentation tools, document sharing and works across all platforms
Big Marker - BigMarker offers free unlimited webinars and web conferences with up to 100 attendees - web-based - up to 12-way video conferencing and screen sharing with recording facility
Google+ Hangouts - Web-based - Video conferencing for up to 10 participants