Mac OS X:

-MidiMonitor is a utility for Mac OS X which displays MIDI signals in a variety of formats. It can watch both incoming and outgoing MIDI streams, and can filter them by message type and channel.Musicians will find it handy for tracking down confusing MIDI problems. Programmers can use it to test MIDI drivers and applications.

MIDI Monitor is FREE to download and use. The source code is available as Open Source (under the BSD license).

-SysExLibrarian is a utility for Mac OS X which lets you communicate with MIDI devices using System Exclusive messages, also known as SysEx. You can record the SysEx Stream for debugging. e.g. to get the clip Name of the Ableton Liveset.

SysEx Librarian is FREE to download and use. The source code is available as Open Source (under the BSD license).

Windows:

For Windows users, MIDIOX is a handy free download that will allow you to view incoming and outgoing MIDI data.  The program is available at http://midiox.com/.  This is also the place to get MIDI Yoke, a loopback (virtual midi cable) that allows 8 sets of I/O.  The two programs are useful in conjunction.  For example, if you would like to monitor your MIDI output from Ableton Live but still be able to pass the data on, you could set Live to output via MIDI Yoke, then choose the MIDI Yoke input in MIDIOX, view it, and MIDIOX will pass it on to the actual output you would like to use.