A number of games/game engines have been released to the public under a variety of licenses. Below are some of them that I have collected, more to look at the structure than anything else. For the most part only the engines have been released without the copyrighted art assets, but some others include the default assets.
Micropolis – based off the UNIX port of SimCity, this is the original TCL/Tk edition of Micropolis for Linux systems, released under GPLv2 (terms included in download archive). On Windows you will need a program that can extract TAR files, I use 7-Zip ( source )
GemRB – this is an open source implementation of the Infinity Engine which can be used to play the original versions of Baldur’s Gate, Icewind Dale and Planescape: Torment. The version I’ve been using is 0.7.2 and haven’t had any major issues whilst playing Baldur’s Gate 2. ( source )