I was browsing reddit :/ when I came across this thread in /r/gaming:

http://www.reddit.com/r/AskReddit/comments/1l31r3/what_is_a_free_pc_game_that_you_think_everyone/

Its about what free games do you think that every one should have, and I found some interesting tittles:

Cave Story:
Its a Japanese side scroller that has been translated to English.
"Cave Story is really one of those games that can be considered a masterpiece of game design."
http://www.cavestory.org/info_about.php

Dwarf Fortress:
"Incredibly deep game where you are in charge of a colony of dwarfs and is presented in text-only Code page 437 graphics though there are mods for better visuals. You control EVERY aspect down to the tiniest single member of your fortress."
Although I have heard that its not for everyone.
http://www.bay12games.com/dwarves/

I haven't had the time to try these [Exams, revision, assignments... ugh] although I'm sure some of you retro fans have, but in case you haven't check them out, there's no reason not to...THEY'RE FREEEEEeeeeeeeeeeeeeee