FreeBoardGames.org is described as 'free and open-source board game platform. Enjoy free high-quality games on any device that can access the web. Study how the games are made, change them, and contribute back to the community' and is a Board Game in the Games category. There are more than 10 games similar to FreeBoardGames.org for a variety of platforms, including Windows, Online / Web-based, Mac, Linux and iPad. The best alternative is Tabletop Simulator. It's not free, so if you're looking for a free alternative, you could try Board Game Arena or Yucata. Other FreeBoardGames.org like games are Tabletopia (Freemium), Vassal (Free, Open Source), Boardgame Lab (Freemium) and Battlegrounds Gaming Engine (Paid).
Tabletop Simulator is the only simulator where you can let your aggression out by flipping the table! There are no rules to follow: just you, a physics sandbox, and your friends. Make your own games and play how YOU want! Unlimited gaming possibilities!
With Board Game Arena you can play board games online with the whole world. It is a next generation game platform which makes online gaming simpler and more attractive: -- Nothing to install: play straight from your browser, from anywhere, at anytime.
Vassal is a game engine for building and playing online adaptations of board games and card games. Play live on the Internet or by email. Vassal runs on all platforms, and is free, open-source software.
Battlegrounds Gaming Engine (aka BGE) is virtual tabletop software for playing boardgames, wargames, card games, and dice games, whether your players are in the same room, or on the other side of the world.