This is an updated version of WhatsOpen.app v2.1, by Franklin Marmon, released around 2008.
This re-release is a 64 bit app, so that it runs on macOS 10.15 Catalina and later.
See here for a few sites explaining the original app:
Should you have an interest in having these features brought back, then please contact me if you can supply me with a copy of version 2.5 or later.
Current version: 4.0.1, requires Mac OS X 10.10 or later (Release Notes)
The source code (Xcode) can be downloaded at github
Thanks to the original author, Franklin Marmon, for giving me permission to re-release the application here.