A base mesh is a low-resolution polygonal model that can be used as the starting point for digital sculpting.
Creating a Base Mesh
When using a sculpting app it's usually best to create a cage or basemesh that is as close as possible to 100% quads. Also try to avoid "poles" of 5+ edges running into each other. Triangles and poles can lead to "pinching" in the model when sculpting.
Keep the quads a fairly uniform density over the model, and be clever about adding more detail where it will be necessary (eyes, ears, nose, etc.), since otherwise you will end up subdividing your mesh far more than necessary just to get enough polygons to sculpt detail into these areas.
It helps to add a quick UV to the basemesh before sculpting, since this can be used to bake a DiffuseMap if you choose to do some 3D painting at some point. This is much easier than trying to UV your basemesh later and trying to morph it or merge it with your sculpt. For methods see the Polycount thread Importing UV's into sculpted mesh in ZBrush.
Some more good advice from Scott Eaton in his ZClassroom article Artist In Action: Digital Figure Sculpture.
Base Mesh Library
If you use one of these as the base for your sculpt, please credit the author. For example "Sculpt by me, base mesh by Samuel Sharit".
These meshes are made specifically for digital sculpting, by avoiding triangles and n-gons wherever possible. For better in-game mesh wireframe examples, see the Topology pages.
The meshes are in OBJ format unless otherwise noted.