midimon is a free command line MIDI monitor for OS X. You can monitor any MIDI device connected to your Mac, or another application's MIDI output by using midimon's virtual mode. midimon should work with any Mac running OS 10.4 or later.
farmpad is a free application for OS X which lets you customize how you use your Korg padKontrol MIDI controller. You can use pads as toggles that light up when switched on, the setting buttons can be used as extra switches and triggers, and the settings dial can be used as a rotary encoder. The controls can be customized using config files. A Windows version is planned for the future once the features in the OS X version are complete. farmpad should work with any Mac running OS 10.4 or later.