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.