The HFC Base theme (originally HFCCBase) is a starter theme originally derived from the Drupal 5 version Basic theme by Raincity Studios. Since that time, these two themes have diverged widely, as new features have been added to Basic, while this theme has gone its own way.
The HFC Base theme is starting point for Drupal theme work at Henry Ford College and for other sites I've built outside of HFC. When creating a sub-theme, themers will generally want to copy main.css and overwrite it completely, as this is where most of the visual styling occurs.
6.x-1.x Branch
This branch is basically the Drupal 6 derivative of my original theme, and didn't have too many fundamental changes to its structure over the years. I have several production sites running with this theme as its base.
6.x-2.x Branch
This version was a major overhaul. I cleaned up a lot of cruft without worrying about supporting existing sites, and changed the templates over to a responsive, mobile-first layout using the Gridless HTML5 and CSS3 boilerplate.
7.x-1.x Branch
This is a straight port from the 6.x-2.x branch, but I ripped out a lot of stuff because much of what the old theme did is now provided by core. The biggest thing this is missing is the username formatting that was based on Drupal 6 core Profile module. Other than that, it's basically ready for use.
1.0.x Branch
The 1.0.x branch for Drupal 8+ is being maintained as relevant changes are made to HFC's Kingfisher base theme. This theme is built on stable9 and is intended to be a replacement for classy.