For anyone interested in an alternative to Roll20, Fantasy Grounds, Astral or other virtual tabletops designed for roleplaying games, Foundry is a one-off purchase with an active development cycle, an enormous library of free community-contributed add-ons, and is becoming very well supported by mainstream publishers making specific Foundry-compatible content available. It is unquestionably one of the best options in the VTT field.
Only one person needs to own the licence, allowing the program to be run on their own computer, set up on a dedicated (subscription-based) hosting service or, with a little extra work and technical understanding, on cloud platforms such as Oracle Cloud. Clients connect using a web browser, as they do for most other VTT options.
There is a demo of the program on the home page and lots of tutorials, support guides, content reviews and commentary on YouTube.
If you are looking for a good VTT platform, this is thoroughly recommended.
I'd second this for anyone interested. It is one of the most feature rich and customisable VTTs I've used. Would highly recommend. Warning, you can get lost in some of the additional features that you can add to it!