Dark Side is the 1988 sequel to Driller is a very early example of a first-person, full 3D, explorable world, that can be viewed from almost any angle.
The game engine – Freescape – has gone on to become famous as one of the earliest examples of its type, and one that was hugely influential on every 3D exploration game that followed.
Dark Side – in my opinion – is much more interesting than Driller, the game that preceded it, and in it you play a government agent sent to an alien moon to fight terrorists and to disable an immense beam weapon they have built, before it is used on your home planet.
You can walk around, fly (using a jetpack), shoot, and rotate your angle of vision up or down, or around.
Considering the Spectrum‘s limitations, Dark Side is a tremendous achievement. That such a complex game was crammed into just 48K of memory is quite staggering really.
A number of other Freescape games followed Dark Side, courtesy of publisher Incentive, though you could argue that none of them managed to better this game.
More: Dark Side on Wikipedia