Minecraft - A block-building game that allows you to create and explore entire worlds from scratch.
OpenTTD - An open-source clone of the classic game ...
Banished - As always, Email support (at) shiningrocksoftware (dot) com if you have any issues. This will likely be the last update of Banished, except for any major bug fixes. The OSX and Linux versions development has stopped, and won't be released.
Micropolis - Micropolis is the open source version of SimCity Classic from Maxis, developed by Will Wright and Don Hopkins.
Terraria - Dig, Fight, Build! The very world is at your fingertips as you fight for survival, fortune, and glory.
Simutrans - Simutrans is an open-source transport simulator.