http://www.uml.org/
http://en.wikipedia.org/wiki/Unified_Modeling_Language
http://ootips.org/uml-hasa.html
http://www.agilemodeling.com/style/stereotype.htm
http://www.agilemodeling.com/style/classDiagram.htm
http://projects.gnome.org/dia/umltut/index.html
http://www.sparxsystems.com.au/resources/uml2_tutorial/
http://www.sparxsystems.com.au/resources/uml2_tutorial/uml2_classdiagram.html
http://www.ibm.com/developerworks/rational/library/content/RationalEdge/sep04/bell/
http://devmentor.org/references/uml/class_diagram.html
http://www.holub.com/goodies/uml/
http://www.tutorialspoint.com/uml/index.htm