2 years ago, I learned of an open-source project called Graphics Gremlin (GG) by Eric Schlaepfer who runs the website Tubetime.us. It is an 8-bit ISA graphics card that supports display standards like Color Graphics Adapter (CGA) and Monochrome Display Adapter (MDA). CGA and MDA are display standards used by older IBM(-compatible) PCs in the 1980s.
The frequencies and connectors used by CGA and MDA are no longer supported by modern monitors hence it is difficult for older PCs of the 1980s era to have modern displays connected to them without external adapters. GG addresses this problem by using techniques like scan doubling (for CGA) and increasing the vertical refresh rate (for MDA) then outputing to a relatively newer but still old VGA port.
As neat as this project is, it does have a few limitations that the author tried to address: it doesn’t have modern outputs, which is becoming problematic with monitor makers no longer adding VGA ports, and it can’t display on two outputs at once. This article details his solutions.