Ammo Scavenger

Description - Increases the number of shots per ammo clip.
Tips - Use this on your weapon of choice. Sniper Rifle, Rocket Launcher, Mag Rail, SMG, and Shotgun come to mind.

EMP Converter

Description - Charges shots with an electromagnetic pulse which damages bots, cameras and turrets. EMP shots drain more ammo.
Tips - Its sometimes cheaper to just use EMP Grenades, Remote Drone Biomod, or sneak past rather than using this mod. If you have the Mag Rail use its alternate fire instead of this mod.

Fragmentary Round

Description - Causes shots from ballistic weapons to explode into fragments upon impact, inflicting radius damage.
Tips - Combine this with the SMG and you get a fireworks display.

Glass Destabilizer

Description - Vaporizes glass with ultrasonic vibrations -- breaks the glass silently and does not trigger alarms wired to the glass.
Tips - Apply this to a silenced weapon, like the Boltcaster, and you have a stealthy way of breaking and entering.

Increased Damage

Description - Increases the damage inflicted by each shot fired.
Tips - Don't bother using this on the Sniper Rifle. Once you've zoomed on your target's head they only take one shot. Well, most targets anyway.

Increased Range

Description - Increases the firing range of a weapon.
Tips - I didn't find much use for this mod. Long range weapons don't need it and short range weapons either got tossed or were melee which can't be modded.

Refire Rate

Description - Increases weapon refire rate.
Tips - Mag Rail, Sniper Rifle, and Boltcaster are some good choices for this mod.

Silencer

Description -Dampens weapon sounds, including those of firing and impact. Once installed, will affect every shot fired from that weapon.
Tips - Sniper Rifle is a must have for this mod. Your effectiveness as a sniper increases considerably when the enemy doesn' know where you are.