Requires Palm OS version 5.0 or above, less than 5kb.
Supports Sony CLIE's Jog/Back buttons as well.

Enabled by default, it can't get any easier than that:
Hold first hardware button for 8/10 of a second and release to HotSync. (You'll hear a confirmation sound after holding the button long enough)

Disable by clearing the "Hold..." check-box, or change the button and the hold period to your liking.

Best of all, you won't lose a precious button for this:
Pressing the button shortly, does the default action for that button.
Holding the button when device is unplugged, should do nothing at all.

So simple, so useful - how come nobody thought of this earlier ?!? ;)