David Richardson has created a multiple-part tutorial for modeling a mech in ZBrush.
Ben Mauro - ZBrush for Environments
Ben Mauro has released a free 3D environment design tutorial using ZBrush, Keyshot and Photoshop.
Cezar Brandao - Sculpting Livestream
Take a look at Cezar Brandao's sculpting-centric Livestream channel.
Cyborg Design - Concept Art Production Techniques
Instructor Michael Pedro gives a detailed overview on concept art production techniques in ZBrush and Photoshop.