What I have not included on the list are multiplayer-only games, browser-based games, and all kinds of free-to-play and "freemium" online products. I have also tried not to include projects that make use of original assets like graphics and sounds (unless authorized by original copyright holders, as is the case of T2002), so games like Mortal Kombat Project, Gods Deluxe or Return of the Triad are not on the list. Another thing out of the scope of this list are engine recreations that require original game assets to work.
Any further suggestions as to what games qualify as free alternatives are more then welcome

Age of Empires Anno series Battle Isle 2200 BoulderDash Castlevania series Chaos Command & Conquer Command & Conquer: Renegade Crimsonland Dark Colony Diablo Doom Doom II
- HacX L
- Realmz L
- Xargon L
- Ancients I: The Deathwatch S
- The Infernal Tome S
- Yendorian Tales Book I: Chapter 2 S
- Yendorian Tales: The Tyrants of Thaine S
- Excelsior Phase One: Lysandia S
- The Fantasy Worlds of Tamrak S
- Hera: Sword of Rhin S
- The Kingdom of Syree
- The Kingdom of Syree II: Black Magic S
- The Kingdom of Syree III: The Depths of Hell S
- Nerakian Adventure S
- The Rescue of Lorri in Lorrinitron S
- Silmar S
- Sword Quest 1 S
- Untima IX: Descension
- The Walls of Bratock S
- Zorak: The Mountain of Terror
- Flame Storm: The Bratockian Saga part II S
- The Great Ultizurkian Underland S
- Joyous Rebel L
- Madman
- Ultizurk III: The Guildmaster's Quest S
- Wraith S
- Yendorian Tales Book I S