Take Extreme Programming, add modeling and remove programming and this is what you get.
A methodology devised to fit a market need.
What is Extreme Modeling? --http://www.extrememodeling.org/whatisxm.html
Extreme Modeling for me is creating models which embrace change. I create models to change daily and to be thrown away when they have served their purpose. This agility is attained by keeping them light weight, capturing only the essential aspects of what is trying to be communicated.
I use my own modeling language which I call CasMl (designed to Communicate Architect and Simplify). As the models change daily there is no concept of completeness. UML is designed to be 'correct' and specificational. CAS ML is designed to be 'good enough' and effective in communicating.
An Example can be seen at: http://www.extremeArchitects.com/architecture/BekinsSTS.htm [This link is broken: can we please have another?]
Q. How can you recognize an extreme modeler?
A. Ask them to change their models. If they smile and ask 'what do you think we should change?' - they are. If they defend their models and modeling technique - they are not.
See also: AgileModeling, TestDrivenAnalysisAndDesign, HowToCombineExtremeProgrammingAndModeling