afni.nimh.nih.gov

Besides Rick's suggestion for pen mode, on Mac OS X, you should be sure to turn off "emulate three button mouse" in the X11 preferences. If that is turned on, holding the option button down while clicking with the left button will emulate the middle button. If you are using a proprietary software driver, from Logitech or USB-Overdrive for instance, you will also need to make sure any mouse drivers are configured to use the middle mouse button, and the middle button is configured to be button 2.