Factorio is developed by Wube Software. They do not offer the full game for free permanently. However, they offer a free, unlimited-time demo and occasional free weekend events. The links below point to the official demo or authorized mirrors.
Steam will automatically "downgrade" your files to version 0.16.51. Official Demo Factorio 0.16.51 - Windows- Linux- Mac Free Download
This version is widely considered the last major "experimental" milestone before the massive 0.17 and 1.0 overhauls. It represents a specific era of gameplay balance and mod stability. Factorio 0
is a significant milestone in Factorio’s development history. Released in early 2018, this was the final stable build of the 0.16.x experimental branch —an update that introduced some of the most beloved features in the game before the 1.0 release. The links below point to the official demo
A complete overhaul of the terrain system, featuring Cliffs as natural barriers and a new map generation preview.