Stellar Forces is described as 'free turn-based multi-player squad strategy game'. There are three games similar to Stellar Forces for a variety of platforms, including Linux, Mac, Windows, PortableApps.com and Android. The best alternative is The Battle for Wesnoth, which is both free and Open Source. Other Stellar Forces like games are TripleA (Free, Open Source) and Advanced Strategic Command (Free, Open Source).
TripleA is a free (under GPL) turn based strategy game based on Axis & Allies board game. A wide variety of map scenarios have been developed for TripleA (World War II, Punic Wars, Napoleonic wars, etc.
Advanced Strategic Command is a free, turn based strategy game. It is designed in the tradition of the Battle Isle series from Bluebyte. ASC can be played both against the AI and against other human players, using hotseat or PlayByMail.