1. Gunmetal

An amazing late-90s DOS game where you pilot a futuristic hovertank in service of a megacorporation. It's very fun, and there are two demo versions available with different levels.
2. SPWW2

A free wargame built with the code of SSI's classic Steel Panthers II. An updated Windows version is available (external download).
3. Star Control II

Join the fight against the oppressive Ur-Quan and liberate your homeworld in this strategy/adventure title. An open source version running natively on modern systems is available as well as an HD remake with updated graphics (external downloads).
4. Fallout

The one and only Post-Apocalyptic adventure role-playing game. The demo is for Windows only, but you can get the full version from GOG.com.
5. Magic Carpet 2: The Netherworlds

The sequel to Magic Carpet features numerous improvements, more magic, more monsters, nighttime and cavern levels, and a whole new set of worlds to rescue of course. There are two different demo versions, and the full game is available from GOG.com.
6. Carmageddon

Everyone's favourite free-for-all, no-rules-of-engagement racing game. Play the demo of the original or Splat Pack, or get the full experience from GOG.com.
7. Sango Fighter 2

Conquer Ancient China! The sequel to the original Sango Fighter gives you more characters to play as and a more in-depth story mode with strategic elements.
8. Turrican II: The Final Fight

The platform side-scrolling shooter legend ported to DOS. Along with the playable demo, you can also get an authorised remake and an inspired sequel (external downloads).
9. Metaltech: Earthsiege

Save the Earth from human-created Cybrids piloting combat mecha robots. This game has been released as freeware (external download).
10. Tricky Quiky Games

A very high quality non-violent platform game intended to promote Nesquik products. Fun for all the family, and besides, it's free!