ZBrush

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