One of the most effective ways to get students collaborating with each other is to use tools that are easily accessible. If the tools are browser agnostic, have no demand for installations or updates, and are intuitive, the collaboration is more likely to be effective.
I recently presented this slide show and Google Doc at the 2011 NJEA Teaching and Learning Symposium.
Here is a link to a Google Docs with all of the sites we discussed and tools to help facilitate global collaboration: https://docs.google.com/document/pub?id=1VckCWWALUyOjjKMAphzL1-YOpl9ULnLFegALAW7V6z0
Many thanks to Lisa Thumann for asking me to present and helping out with resources.