Stencils tagged with ‘use case’
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.
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.
A Stencil with ordinary UML Use Cases with some color added. Note: This stencil uses the Omnigraffle Pro Subgraph feature.