See the following IEEE standards:
     829-1983 IEEE Standard for Software Test Documentation
     1008-1987 IEEE Standard for Software Unit Testing
     1012-1986 IEEE Standard for Software Verification and Validation Plans
     1059-1993 IEEE Guide for Software Verification and Validation Plans

You can contact the IEEE in the US at 800-678-4333.  Also, Requisite
(800-732-4047) sells a Standards Pack with Microsoft Word Templates for
829, 830, and 1012 (part number 3100-05879).  The Software Productivity
Center (<http://www.spc.ca>, 604-662-8181) sells Word templates,
including a test plan standard and a test plan template.

Several people recommend Kaner's book, _Testing Computer Software_, for
test plan samples.  You'll find the ETET test plan in
<ftp://ftp.ecs.soton.ac.uk/pub/etet/> under the path
"doc/testplan".  See <http://www.acomtech.com/testplan.html> for an
adaptation of IEEE 829.  And visit
<http://members.tripod.com/~bazman/frame.html> for a sample system test
plan.  There is an outline of an IEEE based test plan on
<http://www.itc.nrcs.usda.gov/sqa/tst_outline.html>.  

StickyMinds.com also have a template section for test documents at
<http://www.stickyminds.com/swtest.asp?btntopic=tem&Function=Search
&topcat=SWTST>.

Information in this section was gleaned from posts by: Burt Gearhart,
Wayne Woodruff, Brent Parsons, Castor Fu, Michele Mercer, Barry
Dorgan, and others.