Red Alert. Ok, you can also grab Command & Conquer: Tiberian Sun, Tiberian Dawn and that Firestorm expansion, but we all know it's Red Alert that all pure RTS fans will enjoy.
Wing Commander Saga. Haven't played it as much as it deserves just yet, but it really feels Wing Commander-y enough, despite using the freespace-y Freespace 2 engine. And remember to always give your wingman a hug.
Oolite: Should Wing Commander fail to tickle your space exploring instincts, Oolite should probably be what you're looking for. It's the best update/remake/expansion of Elite possible.
King's Quest III. An excellent remake of a Sierra adventure classic. Comes complete with a point-and-click interface, lovely VGA graphics and a voicepack. Still a tough one to beat, mind.
Squid and Let Die. Well, it does feel a bit like Pac-Man, but it also reminds me of octopi, a retro-futuristic version of the 80s and shmups. Excellent fun!
Cactus Arcade. It's the multicart of the modern PC gamer with a thing for indie things. 17 brilliant games are included and everything -from shmups to adventures- is happily being given a chance to shine.
The Elder Scrolls II: Daggerfall. The first truly brilliant game in the rather epic Elder Scrolls CRPG series. Beards optional.
Every day the same dream. It's a webgame of sorts and it's absolutely fantastic. Not particularly fun mind, but something you -YOU!- got to play. By Molleindustria.
Planet MULE. Based on the C64 multiplayer strategic classic, this is a fantastic race for space colonization with beautifully remade graphics and sounds.Related @ Gnome's Lair: