Circa 1995: The venerable X-com (also known as UFO: enemy unknown) was the first squad level tactical game that became a commercial success. The first squad level tactical game was called Laser Squad(very enjoyable game for me as well); while some can say it's also good, it never came close to the sheer playability and fun factor that X-com has.
Now everyone can once again play this classic strategy game by way of abandonware sites. These abandonware sites are dedicated to old games that are in the public domain or games that are so old they are no longer sold anywhere.
You can visit Abandonia for a more extensive review of X-com. This site also has the downloadable Collectors Edition version of X-com that can run in Windows. For alternate downloads you can visit BHlegend for the Dos 1.0 version of X-com. For all things X-com(patches, utilities, strategy guides), visit Xcomufo.