Yuuguu is an interesting tool. It is a free (there are premium services that do cost) tool that once downloaded allows PC, Mac and Linux users to share their computer desktops with each other. The free account lets you share your desktop with up to five others at a time, while also sharing control of your keyboard and mouse.
Yuuguu also has a chat function and conference calling (conference calling incurs a small fee). Yuuguu could be a good tool to teach others how to use specific Web 2.0 programs or for country or remote schools to communicate with other schools or regional staff.