UML

The Unified Modeling Language exists as a standard for making illustrations of system behaviors. In software engineering, UML has the role of illuminating difficult system interactions and serves as a tool for working out complex sequences or dependencies. Many tools exist for UML design, but many people still prefer good old pencil and paper.

Tools:

• argoUML: free and usefull
  
• IBM's Rational Rose: not free
  
• JUDE: not free, but has 20 day trial.
  
• UMLet: free *Drawing* tool for uml
  
• Poseiden for UML: free for 30 days
  

More ideas here? Drop a comment.