This tutorial appeared in the SIGGRAPH 2002 course on NPR. A revised version appears in the July/August issue of IEEE Computer Graphics and Applications 2003. The SIGGRAPH version has a more comprehensive list of references (as of Spring 2002).
CG&A 2003 tutorial: PDF (4.5 MB)(shorter, revised, and slightly more readable)
older SIGGRAPH 2002 version: PDF (10 MB)
(contains many more references)
reference: Aaron Hertzmann. Stroke-Based Rendering. In K.-L. Ma, editor, Recent Advances in Non-Photorealistic Rendering for Art and Visualization. SIGGRAPH 2002 Course Notes.
Here is source code for the rendering strokes in OpenGL.