Abuse hints & tips
Here are some miscellaneous hints and secrets to help you play the game.
Shoot those rockets down
Grenades and seeker missiles can be shot in midair to cause them to explode prematurely, which can make for a good defense; your basic laser gun is fairly good for this when charged up with ammo. Beware, though... the same is true of your own grenades and rockets so you might want to think twice about using those weapons against things like rapid-fire turrets.
Apply blast damage through solid architecture
The blast damage from rockets and grenades can damage things on the other side of a thin enough floor/ceiling. This typically costs more ammo than scoring direct hits does, but sometimes it's worth it to blow up a large cluster of enemies while they're still safely walled off from reaching you.
Destroy Ants with exploding architecture
If an enemy is in contact with a destructible wall/floor/ceiling when it explodes, it will be instantly blown up as if with napalm. In some areas this is helpful for destroying hordes of Ants at once. It works on other enemies as well, but tends to be most effective against Ants due them usually crawling directly on a floor or ceiling.
Destroy enemies with their own turrets
Gun turrets don't only kill Nick. They'll destroy all sorts of other things as well, such as Ants and other turrets, which can sometimes be used to your advantage.
Hold down buttons to do things quickly
You can hold down the [down arrow] key while running about or in freefall and you will activate any activatable thing you pass near. This is quite helpful towards the ends of the game where you might need to enter a door as fast as possible to escape a time bomb explosion. Doing the same with [right mouse button] when you're about to get one of the special powers will make you immediately start running/flying when you get the powerup which can also be handy.
Switch to laser without ammo
You can still fire the basic laser without ammo, just at a slower firing rate. However, the game doesn't let you switch to the laser with the  key if you have no ammo. Hitting [CTRL] to cycle weapons instead will allow you to select it even without ammo, though! Handy to know especially for the following...
Secret hunting with lasers
You can use your basic laser gun to check for most secret walls. If the impact of the laser is a white pulse, the wall is typically solid. If you see red sparks instead, it's an indicator that the wall can be blown up if you keep shooting. Be aware, however, that some walls explode only by scripted triggers.
Save when low on health
When you restore a save, your health will be back at full, so if you reach a save station when you're low on health, save! This also means that it may be worth rushing past an attacker if the next save point is just ahead, and clearing the opposition only after you've secured the station. If you die after, you'll be able to hit the ground running when you go back in. When dealing with some of the more extreme custom levels out there this may be the only way to survive.
Welcome to warp zone
On the first level (tutorial level), right after the first save station and before you pick up the speed booster chip, you will notice a damaged ceiling. Blow it up and use the jump booster to enter the secret area where you will find a door leading to three exit portals. These portals, from left to right, will take you to level 2 (skipping the rest of the tutorial), level 3 or level 4. (This works for both original Abuse and "Frabs").
Not really a full level, but a unique hidden area. When you reach the exit of level 14, blow up the wall to the left of the exit portal. You will find the entrance to a red, alien area unlike anything else in the game. (Only applicable for retail Abuse.)
Getting music to work
If you are running the game via DOSBox, you'll generally have no problem with the emulated MIDI device. On GNU/Linux, you'll probably have to install and configure a MIDI synth such as FluidSynth on your system before it will work, but Windows systems should have one built in.
If, however, you're trying to run the game natively with music, be aware that Abuse is unusually persnickety about what MIDI hardware it will support. If you don't have a dedicated MIDI unit, a Sound Blaster AWE32 or AWE64 should work if its MPU-401 capability is properly set up (my particular one is the AWE64). You can not get music working on just a Sound Blaster 16 to the best of my knowledge.