SnapYap.com looks like a nice free tool for sending video messages. It doesn't involve downloading any software as it uses the Adobe Flash plugin. You just have to allow access to your webcam and microphone.
You can either exchange messages with other registered users, email links to your messages to people or send them a link to your messages page.
This video from Snapyap.com explains it all.
I haven't tried this yet, but I think it could be quite an interesting communication tool and a nice way of developing materials for students and it's free!
Let me know if you have any luck using it.