X-Com: Ufo Defense and X-Com: Terror From the Deep
UFO Defense was voted the best game of all time by IGN. I personally vote X-Com TFTD as the best game ever made. You have to get over the fact that it's really old, but it's certainly worth playing in the 21st century, too.
In my country, it's legal to download this game if you own it. Last thing I checked, you can buy it with Steam as well.
Thanks a lot to starwindz for releasing this Total Pack!