Burning Suns can currently be enjoyed for free on the following platforms:
Tabletop Simulator allows you to play Burning Suns in a 3D environment with all components and figures, and complete freedom of movement and actions.
Everything in the game is scripted and fully automated, so that you can focus on strategies and having fun.
Link to Burning Suns on Tabletop Simulator
Tabletopia allows you to play Burning Suns in your browser.
Tabletopia has a 2, 3 and 4 player setup which includes Terrans, Changelings, Templars, Cyborgs and Guardians.
Link to Burning Suns' Tabletopia Game Page
Vassal is a free open-source game engine for playing online adaptations of board games.
You can play Burning Suns either live on the internet or by email. Vassal runs on all platforms.
Link to the Burning Suns module on Vassal
Burning Suns also exists as a Zun Tzu Module, but it's currently out of date, as it's only a prototype - Link to BGG thread.