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.)
The PostScript output is of such a simple form that MetaPost output files can be directly included in PDFLaTeX documents.
Much of MetaPost's source code was copied from Metafont's sources with Knuth's permission.