Shane Olson's stuff and other useful tutorials

https://gumroad.com/shaneolson

This is a great set of videos but takes 9 hours to watch all together
https://www.youtube.com/watch?v=J2B0x9TkFuA&list=PLkzopwqcFevYqrk_0MKIaUwrWYILzYsp6
https://www.youtube.com/watch?v=DHrLa2IE8ug&list=PLkzopwqcFevbwRUsPn13Pu8ZmCvIej9qm
https://www.youtube.com/watch?v=Op-eDAt1GlA&list=PLkzopwqcFevZsRJF83oqGGgszxLk83Rsg

Zbrush's official classroom site
http://pixologic.com/zclassroom/homeroom/