The very first weapon you'll encounter, the Wrench is, despite its humble appearance, actually one of the most useful weapons in your arsenal. Not only does it deal a reasonable amount of damage at close range, it has a utility function in that it can be used to smash through barriers. Additionally, it can be augmented by O/S Upgrades like Lethal Weapon, and Psi Powers like Adrenaline Overproduction. And the more you pump up your Strength, the better it gets.
The Wrench will in all probability form the backbone of your arsenal for much of the game. Ammunition early on is scarce, and the Wrench actually deals more damage than the Pistol in most situations. To use the Wrench effectively, you'll need to get the hang of circle-strafing enemies and using hit-and-run tactics, but once you do, you'll find even Cyborg Assassins won't be able to stand up to your beating.
Just note that while it's possible to play 99% of System Shock 2 using only the Wrench, you won't be able to defeat the game's final stages without at least a Pistol.
Standard Attack, Power Attack (with Smasher O/S Upgrade)