The MetaPost system (by John Hobby) implements a picture-drawing language very much like that of Metafont except that it outputs Encapsulated PostScript files instead of run-length-encoded bitmaps. MetaPost is a powerful language for producing figures for documents to be printed on PostScript printers, either directly or embedded in (La)TeX documents. It includes facilities for directly integrating TeX text and mathematics with the graphics. (Knuth tells us that he uses nothing but MetaPost for diagrams in text that he is writing.)
Although PDFLaTeX cannot ordinarily handle PostScript graphics, the output of MetaPost is sufficiently simple and regular that PDFLaTeX can handle it direct, using code borrowed from ConTeXt - see graphics in PDFLaTeX.
Much of MetaPost's source code was copied from Metafont's sources, with Knuth's permission.
A mailing list discussing MetaPost is available; subscribe via the TUG mailman interface. The TUG website also hosts a MetaPost summary page.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=MP