Having an admiration for the effort that went into making an attractive and usable human interface, back in the days when screen-real-estate and computing resources were expensive, I've decided to put some effort into replicating the look of older interfaces using the technology of newer ones. All the themes here work on a modern GNOME desktop, such as Ubuntu Linux or Fedora Linux.
All themes are BSD licenced, and can be installed by clicking "Install..." in the Appearance Preferences window then choosing the downloaded theme file.
One day, I hope to offer authentic font-packs to make these themes even more realistic.
Before Aqua, before Luna, before Platinum, there was the original Macintosh operating system, so fundamental it didn't even have a name - it was just called the "system software", and people would refer to "system software version 4" or "system 4" for short. This theme more-or-less faithfully replicates the look of System 7 running in black-and-white mode (but see the included README for caveats, limitations, and translation difficulties)
Download Mac OS Classic B&W, or see a screenshot.
Although Windows 3.0 is the version that first gained popularity, Windows 3.1 had a lot of visual polish that made it much nicer to look at. Still pretty ugly, but much nicer.
Download the Windows 3.1 theme, or see some screenshots: 1 2