Stencils tagged with ‘use case’
Free
UML 2
Additional UML diagram tools, esp. for Use Case and Activity diagrams. There is also a Domain object for class domain or namespace groupings in Class and Collaboration diagrams.
Free
UML Use Case (Simple)
A very basic UML Use Case stencil that I've found useful. Has the basics of what you need to produce quick and simple UC diagrams.
Free
YAUCS - Yet Another Use Case Stencil
A Stencil with ordinary UML Use Cases with some color added. Note: This stencil uses the Omnigraffle Pro Subgraph feature.