One of the online contributions I have made over the last 6 years since I began working in virtualization is to contribute on a regular basis to a number of forums. It takes up a lot of time, but I check these forums most days to see if there are questions/discussions that I can contribute to.
This one is kind of obvious, and I am proud to be a moderator too, most of my work is done in the various Training & Certification areas.
Some great technical discussions in the VMware Virtualization area, the occasional VMware-related post in the Other Certifications area.
A mixture of technical and certification posts in the Virtualization area.
Plenty of certification posts in the VMware Certifications area, technical posts can be found in the Virtual Computing area.
I would highly recommend all of these forums to you - I'm scott28tt on all of them - see you on there!