(from http://edorigami.wikispaces.com/Bloom's+Digital+Taxonomy)

Collaboration is not a 21st Century Skill, it is a 21st Century Essential.

In a recent blog post from the official google blog, Google identified the following as key traits or abilities in 21st Century Employees:
“... communication skills. Marshalling and understanding the available evidence isn't useful unless you can effectively communicate your conclusions.”
“... team players. Virtually every project at Google is run by a small team. People need to work well together and perform up to the team's expectations. ”
Source: http://googleblog.blogspot.com/2008/07/our-googley-advice-to-students-major-in.html
If we look at UNESCO's publication “The four pillars of Education, Learning: The Treasure within” Collaboration is a key element of each of the four pillars.

Learning to know
Learning to do
Learning to live together
Learning to be

(http://www.unesco.org/delors/fourpil.htm) [10]

So to prepare our students, our teaching should also model collaboration. A vast array of collaborative tools are available; wikis, classroom blogs, collaborative document tools,social networks, learning

management systems - Many are available at no cost. If you have not yet tried them, look at:

wikis – wet paint and wiki spaces (or 'Pages' within Canvas)
Classroom blogs – edublogs, classroomblogmeister, blogger
Collaborative document tools – Google documents, zoho documents, adobe Buzzword
Social Networks – ning
learning managements systems

These tools are enablers of collaboration, and therefore enablers of 21st century teaching and learning.