Pixologic’s MatCap Library – a collection of MatCap created by the community hosted by Pixologic.
ZBrushguides.com’s materials and matCaps – a collection of a few dozen materials and MatCaps with nice preview images and names.
Badking.com.au’s materials and MatCaps – a large collection of materials and MatCaps.
Pierre Rogers’s materials and MatCaps on CubeBrush – a collection of 10 MatCaps (not for free).
whats the difference between materials and matcaps?
The short and simple explanation is that matcaps = materials + lighting. This means that if you apply a matcap to your tool, the light configuration in your ZBrush scene will have no affect on that model. To change the light configuration you would then need to edit the matcap itself.