The best real time strategy game ever made (although some purists prefer the original to the sequel). Still playable online thanks to the efforts of www.playmyth.net. As far as getting a copy you'll have to find one used or else pirate it, as far as I know.
If you don't know what Myth is, it's an RTS game that has no base building, in other words, you are given some units and you fight to the death (or for objectives). The graphics are rudimentary by modern standards, but the gameplay is unmatched.