If you have two-finger trackpad tap set to function as a secondary click, you can also just hold two fingers down on the trackpad and click the track button for a secondary click. This works in 10.5 Intel and PowerPC. I'm not sure if it works in 10.4, but I'm guessing it will work fine on Intel and PowerPC.
[robg adds: This method of clicking can be useful if you're using two fingers to scroll; you don't have to lift them and then tap when you want a contextual menu. Just click the mouse button with the fingers still on the pad, and you'll get a control-click.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080115005822329