This download contains the MetaPost macros needed to easily create UML diagrams: metauml_lib.
The MetaUML manual comes with the sources (under GNU FDL)! This means that you can get direct accounts of how things are done: metauml_manual.
MetaUML Library of Patterns
This package contains MetaUML-typset UML class diagrams useful in explaining the classic Design Patterns from GoF: metauml_lop.