Free and Open Source Virtual Table Top software

While there's a lot to appreciate about Roll20 and its integration with the books and character sheets of the most popular RPGs—perhaps it's not for everyone. Good as it is, it could also be considered:

intimidating for players wanting a simpler interface
boring for players that have experienced nothing else
irrelevant to fans of less popular RPGs that aren't deeply integrated into the platform
expensive for GMs that wish they had more power or storage
repugnant to users that prefer FOSS, alternative platforms, or self-hosted options

I may skew into one or more of those categories. But what else is there?

gTove

A lightweight web-based open source VTT that nonetheless offers stacked map levels, and 3D-like rotation of the map to view the table from any angle. Of course, some popular features like animated dice and Fog of War are also built-in. Storage comes from Google Drive integration, but it's necessary for a demo and an 11-step walk-thru. I found this one the easiest to use and understand.

MapTool

A Java-based FOSS option that some claim matches or exceeds Roll20's features, without the limitations. However, there's a learning curve to master it. Watch the tutorials on YouTube while you master the software on your PC. Also, a large community of users are happy to answer any questions you may have. If the game you want to run is a commonly-found one (like D&D, Pathfinder, ShadowRun, nWOD, or GURPS), there's a good chance you can locate someone who's done most of the work developing a framework to get you started faster.

Rolisteam

Earlier in development is this multi-platform VTT spurred by the advent of COVID-19. Still maturing, but with many of the features you expect. Check out the tutorial, then download it for a closer look.