StarCraft is a classic computer game that pushed the real-time strategy genre with its use of three factions that played distinctively different. The download, full StarCraft game and Brood War expansion, has a size of less than 2 Gigabytes, and you get days of fun out of it if you like RTS games and don't mind the antiquated looks and interface of the games. The games have been updated with a patch so that they are playable on modern machines. I have not tried multiplayer, so cannot comment on whether this is also freely available or requires a (free) account.
Even better, downloads are direct and don't require a Battlenet account or registration, and that is also true for playing the single player campaign. Blizzard released the classic game for free for both Windows and Mac computer systems yesterday.
