A colleague of mine (thanks Marc!) came across a very handy mechanism to distribute your ArcGIS Python toolboxes and script tools. You can build your own Python Distribution to encapsulate a custom Python module, accompanying toolboxes and helpfiles and distribute it to your audience, by utilizing the Python Distribution Utilities (Distutils). It also builds an installer (such as “mytool-1.0-win32.exe” for Windows), that will install your module in the site-packages directory.
If you include a subdirectory “esri” in your project (see screenshot), any toolboxes or script tools will appear in the Systems Toolboxes in ArcGIS for Desktop, which is also very convenient.
The mechanism is described in the ArcGIS Python Help (here). The help item neatly describes which steps to take and also which naming conventions apply.