ChordLab 3.6.9 for Mac could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The program lies within Education Tools, more precisely Teaching Tools.
The following version: 3.1 is the most frequently downloaded one by the program users. This program is compatible with Mac OS X 10.6.6 or later. We recommend checking the downloaded files with any free antivirus. This Mac app is a product of RoGame Software.
From the developer:
ChordLab is a unique music application that tackles advanced concepts in harmony. So beware - this is not the generic chord-book app that guitar players use to look up a diagram for an E or A chord. ChordLab deals with harmony as a whole.
You may want to check out more Mac applications, such as Dolce Music Flash Cards, Learn To Draw or Sibelius 6 Student, which might be related to ChordLab.