Standard
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Tools Being Evaluated
Tin: This looks to be quite good tool $700 license and it generates activity diagrams too - http://www.casecomplete.com/Video/Default.aspx?a=1
Tin: If going with Word here's a template - http://www.technosolutions.com/use_case_template.html
Tin: Fairly good and simple use case example - http://epf.eclipse.org/wikis/abrd/core.tech.common.extend_supp/guidances/examples/use_case_spec_CD5DD9B1.html