For those who never heard of this game before, it's a space-themed game that was made back in '92, and was one of the first few legitimate sandbox-style games around. And I mean it; there's nothing that forces you to do anything in a specific order and all storyline-related things have at least 2 or 3 different ways that they can be done, not to mention the galaxy is absolutely huge for plenty of exploration (which is never the same each time since non-storyline related galaxies are generated randomly each new playthrough). The different alien races you run into are all very interesting and the world the game takes place in has a surprising amount of overall depth to it (both in terms of history and development), not to mention it had a number of influences on later games such as Mass Effect. Be warned, however: it's rather difficult to beat the game on your first playthrough without a guide.

Those interested can find the game here:
http://sc2.sourceforge.net/
The game is completely free of charge and comes with an online multiplayer to get into dogfights with friends, so feel free to go nuts.