Janet Gregory has provided a sample test plan she's used on agile projects. (In Word and RTF formats.)

It's common for the words "test plan" to be used for a list of descriptions of test cases. (Thus, you sometimes hear people on agile projects responding to the question "Where's your test plan?" by pointing to the automated test suite.) That's never made sense to me.

When I think of test planning, I think in terms of handoffs. At some point, the code is handed off from one set of people to another. For example, a release is a handoff from the production team to the users. That's the main handoff, but a geographically-distributed, multi-team project might have internal handoffs as well.

Everyone should want the receiver of the code to smile. Testing is the activity of predicting whether they actually will smile. Test planning is making sure that everything is in place for that activity to happen smoothly when it's time for it to happen. For example, machines may need to be bought for configuration testing, and the purchasing process may need to start several iterations before the testing starts.

I wrote a paper about that once (PDF).