Calamus is described as 'A map-making companion for tabletop RPGs, like Dungeons & Dragons or Pathfinder' and is a Virtual Tabletop in the category. There are more than 25 alternatives to Calamus for a variety of platforms, including Online / Web-based, Windows, Mac, Linux and Android. The best alternative is Roll20, which is free. Other great apps like Calamus are Inkarnate (Freemium), Foundry Virtual Tabletop (Paid), D&D Beyond (Freemium) and MapTool (Free, Open Source).
Roll20 is the free, easy way to play tabletop games (such as pen and paper RPGs) online with others. It's completely web-based, so it runs right in your browser. It supports video and voice chat, background music, character sheets, a massive art library, and more!
Inkarnate is an all-in-one fantasy mapmaking platform with a free version and a huge library of assets. With Inkarnate, one can create world and region maps, city and village maps, battlemaps and interiors, as well as scenery maps.
Foundry VTT is a standalone application built for experiencing multiplayer tabletop RPGs using a feature-rich and modern self-hosted application where your players connect directly through the browser.
MapTool is an online, multiuser, networked, graphical, interactive, programmable virtual tabletop. Requires Java 6+. It can be used for face-to-face play, or Internet role-playing in real time even if your players are on the other side of the world.
A fun, easy to use, Open Source tool for tabletop RPG players to manage their characters, campaigns, and parties. The goal is to help you focus more on the game by bringing your paper character sheets into the 21st century.
Standard Action is a tabletop gaming application designed to be simple, but flexible. In game, Standard Action features an interactive game board, live chat, dice rolls, a turn tracker, customizable token stat bars, and much more.
Beyond Tabletop is a set of tools for RPG players with easy-to-use online character sheets and maps. Subscription-free and designed to be used on all devices online. Currently offering a battle map and character sheets for Pathfinder and D&D 5e.