http://www.thegnomonworkshop.com/
http://Crazybump.com
Check out Forums>Announcements>CrazyBump for Schools (it's free for schools)
Pixologic.com
The makers of ZBrush. Check out the Community, Education and Support sections.
http://www.zbrushcentral.com/
An online ZBrush community
http://topogun.com/
Part of a character building pipeline
http://www.3d.sk/
A great place to find reference materials
