Drush Open-Source License
The last version of Drush on drupal.org has a GPL v2 license in it.  The license itself is not in the repository.  Modules and themes on drupal.org automagically get LICENSE.txt added by the packaging script (the automated process that creates the tarballs and zips from central git repo).

This means that drush is technically still GPL v2, even though it doesn't have an explicit license in it.  There is talk about relicensing drush, though.