MatCap (Material Capture) shaders are complete materials, including lighting and reflections. They work by defining a color for every vertex normal direction relative to the camera. You can switch the renderer to MatCap in Scene Settings. You can also switch a Lit model to MatCap in the Model Inspector.
After setting the renderer, you can edit the MatCap from the Materials tab.
The default MatCap is "skin_soft", but there are 6 MatCap images available to all users. Switch between them in the list.
Use the color picker to apply a color to the MatCap material.
Disable Apply to all materials to use a different MatCap for each material. Switch between them in the dropdown.
Use the Curvature slider to increase the contrast of details, including normal maps. It requires smooth-shaded vertex normals on geometries.
Importing Custom MatCaps
Pro members and above can import custom MatCap images by clicking Import MatCap Image. The maximum file size per MatCap image is 4MB.
Free MatCap Resources
You can download MatCaps from lots of different online resources, for example, this library of free MatCaps from nidorx on GitHub.