You might not believe it if you haven't lived through that period yourself, but screen savers were a profitable commercial market for a long time. One of the best-known companies living from this was Software Dynamics, and their prime product was called After Dark - the world's single most popular screen saver. Everybody knew the flying toasters back then.
The very same company made a Shareware game in 1994 (they're claiming 1992 on their website, however, I know for sure it was publically released in 1994). Operation: Inner Space, as it's called, is designed for Windows (a rarity in the days of Windows 3.1), and even more surprisingly, it's an action game. At the first look, it reminds the player of Asteroids: you're flying a spaceship seen from the top, blasting your way through countless screens.
However, there's a twist: This isn't space at all, but the interiors of your own computer. You see, virii have infected your files (yes, your files), and you're taking over the job of cleaning up again. For that purpose, you're traversing your directory tree (the actual one of your own hard drive). Each directory is a 'level' you have to enter with your ship. There, you collect the icons (executables) which aren't infected and destroy the ones which are - and you thought virus scanners are boring mathematical algorithms!
Even that isn't really enough, because the source of these infections lies deeper. In fact, it is yourself - your own 'Inner Demon' which you have to fight / overcome (nice analogy there: if you hadn't installed those virii, you wouldn't have to take the hassle of cleaning up again). By destroying this final boss, you can win the game, too, but that isn't easily done. The Inner Demon can only be destroyed by a combination of several so-called 'Noble Weapons' which you have to earn by special fights before.
Another thing only few people who aren't into computer science are aware of is how much is actually going on within your computer all the time - regardless of how much you're actively doing. To go along with this fact, the 'Inner Space' in the game is anything but empty, too. There are simple obstacles like asteroids (software bugs? hardware problems?), stationary missile launchers (built-in defenses of the software you're trying to clean?) and also other ships (processes?).
These ships are especially interesting. They belong to different 'teams' (fractions), for example, there are the Pirates, who are trying to loot as much as they can. The Renegades are just out to destroy anything for fun. On the other end of the scale, there are the noble and 'good' Knights. Between those (and the ones I haven't listed), there are the Enforcers, the police force of Inner Space. They'll warn, arrest or even kill (if the former two ways don't succeed) ships which aren't following the law.
Depending on whom you align with, the game will be very different. As a pirate, you'll probably find yourself in front of Inner Space's court, where your loot will probably will be taken from you by a high fine again, more often than you like. If you're a very skilled fighter, however, you might stand against the Enforcers and kill even their toughest ships. Or you can just try to avoid or outrun them as soon as they arrive. On the other hand, as a Knight, you'll find yourself under the threat of becoming a victim of those outlaws yourself, and you have to be able to defend yourself. In any case, there is a kind of 'honour codex' in most teams which will make other members of your own team come to your help if you get into a fight (there are exceptions to this, too, though, so choose carefully). To make things even more interwoven, there are alliances and grudges between different teams and also on a personal level.
What's really great about this is that a single ship can never be 'invincible'. Even though you can buy lots of upgrades like better shields, a cloaking device or more / better weapons (which are paid for by collected uninfected icons), you'll never be able to fend off a whole fleet of enemies on your own. This way, the game always stays challenging, even for the most experienced players. Time and time again, you'll have to call the in-game ambulance to get your ship back into one piece again.
Of course, Operation: Inner Space wouldn't be true to the After Dark legacy if it took itself too seriously. The whole game is basically one big gag, but there are numerous visual details to support this. For example, the default weapon of many Pirate ships is huge beer bottles, and at a certain time of the day, the fuel containers which are usually floating around are turning into tea cups. Also, there are fun 'extra' levels to get some change into it - for example, race courses you have to finish first to gain icons.
On the whole, there isn't much which could possibly improved in this game. The premise is simple, yet there are enough details to get it miles above the usual standard of the genre. According to the developer's homepage, it's even running on every version of Windows (again, it was made for Windows 3.1, which wasn't even an operating system, but just a graphical shell). What they're not telling you is that one of the ways to finish the game (collecting all 'good' icons and destroying the 'bad' ones) has become virtually impossible to achieve on computers which are common today. Just think for a moment how many directories (including subdirectories) you have on your hard drive... yup, in theory, you would have to go through all of them. When the game was released, hard drives between 50 and 200 Megabytes were the usual norm - so that is something which isn't practical anymore. Doesn't matter much, though - just play as long as you like and then finish your game by defeating the Inner Demon. It's the better way anyway, because if you work on yourself instead of just squatting the effects on the surface, you won't get infected again in the future (there we are with these analogies again)...