Web browser games
OpenRA is a project that recreates and modernizes the classic Command & Conquer real time strategy games. We have developed a flexible open source game engine (the OpenRA engine) that provides a common platform for rebuilding and reimagining classic 2D and 2.5D RTS games (the OpenRA mods). EA has not endorsed and does not support this product. If you ever played Command and Conquer / Red Alert and liked it, this is for you. They also remade the Dune game based on RA. Multiplayer as well [FREE]
Warzone 2100 is an open-source real-time strategy and real-time tactics hybrid computer game, originally developed by Pumpkin Studios and published by Eidos Interactive. It was originally released in 1999 for Microsoft Windows and PlayStation, and is now also available for macOS, FreeBSD, AmigaOS 4, AROS, MorphOS, Linux, NetBSD and OpenBSD.
Warzone 2100 was developed and released as a proprietary commercial video game, on December 6, 2004, the source code and most of its data was released under the GNU General Public License version 2; the rest of the data followed on June 10, 2008. And its really fun to play! Multiplayer as well [FREE]
Zero-K is an expansive Real Time Strategy game. Fight epic battles over land, air and sea on 100s of maps. Manipulate the terrain and throw units around. Explore an abandoned galaxy, solo or co-op, in 70 campaign missions. Hop online for a tense 1v1 or massive battles with up to 32 players. It’s free and works on most platforms.