X-COM: UFO Defense is a true classic. I played it originally on Amiga for days on end and it has a fully functional open source engine from the community named 'OpenXcom'. The game is free right now on Humble Store, so go grab it.
It's a Windows key, so you will need some form of Wine or Steam commands to grab the data files to use in the open source OpenXcom engine. I've actually put plenty of time into the game inside of OpenXcom and it works damn nicely too.
Don't pass up on a good deal guys.