Subsystems are modules with a passive effect on one or more systems. Note, you can only have 1 of any particular subsystem installed; even if you purchase another one you won't be able to install it.
General[]
Fault Tracker[]
Cost: 19,500
Allows a scanning pulse to find weakpoints in asteroids that mining lasers can hit for extra yield, doesn't work with any other weapon.
Only functions correctly if you're using mining lasers to break the asteroids apart.
Repair Bot[]
Cost: 304,500
Slowly automatically repairs ship systems when damaged but not the hull.
Systems are repaired too slowly to be useful within a fight. Armor is unaffected and is the only ship system that is expensive to repair mid to late game. Only useful if you've got spare subsystem slots available, which isn't very often.
Maneuvering Booster[]
Cost: 963,000
Increases ship turning speed.
Quite a handy addition to a large ship that otherwise turns around like an aircraft carrier.
Ordnance Targeting[]
Cost: 96,000
Allows you to prioritize the targeting of launched torpedoes and mines. Although you can already hit mines and missiles in manual turret control (especially in First Person View), having the Snap-To targeting also trigger on them can be either a blessing or a curse.
When trying to navigate through minefields it can be a blessing, but in a heavy fight you may want your weapons to only target enemy ships instead of the missiles they're firing at you, and just use Deflectors to handle the missile salvos.
Nebula Mitigation[]
Cost: 276,000
Allows your radar to function normally in a nebula.
Without this subsystem, your sensors (the minimap) are erratic while you're in a nebula, which could make it difficult to determine the exact position of enemy ships, cargo containers or rich asteroids at longer range. Equipping this subsystem normalizes it.
Deflection Absorber[]
Cost: 2,808,000
Allows your shield to recharge from explosive blasts like missiles or mines (torpedoes not listed ingame, so their mechanics are probably different).
A sweet piece of gear. Missiles hurt, especially when you've got a whole swarm of fighters launching them at you. This mitigates an awful lot of the pain.
Turret Accelerator[]
Cost: 99,000
Increases the flight speed of projectile turret shots by 25%, which also increases their max range in manual fire mode. Auto fire mode will not take advantage of the extra range from this.
Improves velocity of projectile turret shots, which also inadvertently increases their max range since the shot duration isn't changed (maximum range = projectile speed x duration time). This will still make it difficult to hit fighters unless they're moving straight towards you and larger ships weren't really a problem to hit, so you'll likely get this for the increased range (and given it's barely over a tenth of the cost of the Turret Extender it's well worth it). Has no effect on beam weapons since they're instantaneous.
Heat Management[]
Cost: 101,500
Decreases the heat buildup rate by 10% for projectile and beam turrets during manual firing.
This can definitely improve your damage output by reducing how frequently you have to let the weapons cool down, resulting in a faster firing rate over the long term.
Turret Extender[]
Cost: 927,000
Increases the range of projectile and beam turrets by 25%.
While the ingame description says this subsystem applies to "projectile" turrets, it DOES function with beam turrets. Thats either a bug, or the designers meant to say "non-missile turrets".
Turret Optimizer[]
Cost: 7,897,500
Reduces the reload times for any installed projectile turrets by 25%, resulting in a faster firing rate.
Obviously a handy subsystem if you can afford it, which will only likely occur once you've got enough gear to handily beat the game.
Hacking Spike[]
Cost: 90,000
Increases the time limit that you are given to hack into one of the remote data satellites found through the pulse scanner by a factor of 2.
Missile Extender[]
Cost: 315,000
Increases the range of all missile turrets and secondary missiles by 25%.
Missile Optimizer[]
Cost: 2,740,500
Improves the reload time for all missile turrets and secondary missiles by 25%.
Merchant's Guild[]
Detail Scanner[]
Cost: 95,000
Enhances the information given by a pulse scan including cargo and active bounties on ships.
Gas Scoop[]
Cost: 294,000
Improves booster recharge while in a nebula, Guild faction 3 req.
Mine Repulsor[]
Cost: 2,727,000
Prevents mines from tracking to your ship while your shields are up, Guild faction 5 req.
EMP Ablation[]
Cost: 949,400
Reduces speed disruption effects on your ship by 50%, Guild faction 8 req.
Mercenaries Guild[]
Broadside Accelerator[]
Cost: 94,500
Increases the speed of Broadside projectiles and consequently their range by 25%, Guild faction 0 req.
Broadside Charger[]
Cost: 291,000
Increases the speed of broadside aiming by 25%, Guild faction 3 req.
VERY useful piece of gear. If you're at any sort of range, the limiter to your broadside guns is not their reload speed, but their charge (aiming) speed. You're rarely going to be point blank range on a dreadnaught class enemy, which is the only time you can effectively fire your broadsides faster than they "lock on".
Broadside Extender[]
Cost: 913,500
Increases broadside weapon range by 25%, Guild faction 5 req.
Broadside Optimizer[]
Cost: 3,010,500
Improves the reload speed of broadside weapons by 25%, Guild faction 8 req.
It allows you to spray more bullets/lasers into the void of space, and little else. Would only be useful if paired with Broadside Charger.