X-Com, if you don't know what it is here's the idea: you are a secret governemnt department for fighting alien invaders, when aliens invade a town you lead a squad of guys using the classic Fallout/Fire Emblem style gameplay. A very good game (#12: http://top100.ign.com/2005/011-020.html <-- good article btw)
What made it so great is the ability to capture and even use the alien's stuff, which grew astonishing well with the gameplay, and nearly essesual later on. It was a great memory of my childhood, sayin up till 10:30 will my little brother slowly designing a base. Downside is the game is very old now, if I remember right the whole game was I think 20megabytes (size of about 5songs) with today's tech it could become and amazing game, I am really suprised nobody has done a more recent version of it.