Laurence Finston's 3DLDF is a package for 3D drawing with MetaPost output.
Finomoton allows you to draw a finite state machine interactively and then export it to plain MetaPost format.
Another application package that uses MetaPost is Thorsten Ohl's feynmf for drawing Feynman diagrams.
CERN is publishing a tutorial. Pointers to the tutorials appear on Ohl's home page.
I have also been told that gnuplot can produce MetaPost output.
Numerous contributed MetaPost macro packages appear in CTAN under graphics/metapost/contrib/macros. Another macro package is blockdraw.