I found these days this open-source project which looks very promising, not just because it’s free, but also because it has a lot of features. As the presentation on their website says, VMukti offers:

file sharing, chat, telephony, video conferencing, remote management, CRM and more.

It is built using .NET 3.0/3.5.

I encourage you to download VMukti and vote it on SourceForge as "Best product for Educators"!

Happy collaboration! :-)