This thread is sort of a mix between my cool software thread and K100's ( What games do you play when you are bored? ) Same concept as cool software, im sure there are some good unheard of games out there that not many know about and as a fan of underrated games i would like to know of them.
During my search i found 2 mentionable games that also happen to be free.
Soldat - A nice fast paced side scrolling type shooter that has multiplayer.
Youtube - http://www.youtube.com/watch?v=20xv3Mw4kuQ
Site - http://www.soldat.pl/main.php
Toribash - This is a great little turn based rag doll martial arts game which i though was a pretty simple but unique game, has online play, single player, a mod community.
Here is a youtube video - http://www.youtube.com/watch?v=2FMtQuFzjAc
Official site - http://www.toribash.com/
If i find any more unheard of games i will post them here but in the meantime lets hear your choices.