Various UML Tools support design pattern usage.  You typically drag and drop patterns onto your workspace and they appear as UML classes.  Its a bit more complicated than this, and raises all sorts of interesting issues.  Read the pdf (part 1) and watch the video (part 2).

Part 1 - Early tools and the philosophy of pattern automation

Design Pattern Automation.pdf - Part 1 (pdf conference paper)

Part 2 - Design Pattern Tools as of 2006

A talk entitled "Design Pattern support in UML tools - part II" which I gave on October 2006 to the Melbourne Patterns Group, Australia

Design Pattern Automation - Part 2   (powerpoint)

Video of me demonstrating IBM's Rational tool.