[Hrach_Techie]

This is a good one
http://www.agilemodeling.com/


Use cases
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A task a user will perform using the system.
http://www.agilemodeling.com/artifacts/systemUseCase.htm (note that this one is done textually)
http://www-106.ibm.com/developerworks/java/library/co-design5.html

These most closely relate to end user documentation - What are the steps the user performs to perform the task.

Data Flow Diagrams
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How data (information) will move through the system
http://www.agilemodeling.com/artifacts/dataFlowDiagram.htm

Data Model Diagrams
-------------------
The relationships between data components.
http://www.essentialstrategies.com/publications/modeling/chen.htm

Most useful when developing APIs to ensure that your objects are consistent throughout the system, and to give the user(a programmer) what the relationships are.