[zerokyuu]

Mighty mouse had middle click support; you just click with your finger on the scrolling ball. Not great but you could program what to do with this button.

Unfortunately the Macbook trackpad doesn't have middle click natively. However, with BetterTouchTool you can assign commands for 2, 3, 4, and even 5 finger clicks. For five finger click you have to include the thumb. This means that clicking the trackpad with your thumb while a single finger is on it is a 2 finger click, which I prefer to be a regular left click. These are all settings you can control.

Anyway, BetterTouchTool lets you configure a whole bunch of different actions (clicking or swiping in different parts of the pad with a different number of fingers) to activate a whole bunch of commands.