One of the popular Drupal base-themes you'll find that's being used in multiple / related open-source projects is a humbly-named theme called "Radix".

What is Radix?

Started off as a base-theme for Panopoly distribution, this theme has slowly sprouted into different Drupal distributions (pre-packaged Drupal for specific use cases) such as Open Atrium (common used for setting up intranet sites), Open Academy (for academic organizations), and Restaurant (my favorite one-stop generic Drupal install-profile!)

Radix overview

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Subtheming with drush

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Note: this post is still being edited.