You are Nick Vrenna. It is the year 2009. You have been falsely incarcerated inside a high security underground prison where illegal genetic experiments are taking place.
Alan Blake, the head research scientist, has isolated the specific gene which causes violence and aggression in humans. This genetic sequence, called 'Abuse', is highly infectious, causing horrific transformations and grotesque side-effects. You are the only person to show immunity to it.
A prison riot erupts, and in the confusion all the cell doors are opened. Soon everyone, guards and convicts alike, become infected and transform into a horde of mutants which take over the building.
Your only chance for escape is to don battle armor and reach the control room situated in the structure's deepest level. You must first stop the prison's abuse-infected water supply from contaminating the outside world. Freedom and the fate of the world now depend on you.
The game is notable for its (at the time) unusual control scheme: The keyboard is used to move Nick, while the mouse is used for aiming the weapons. The basic gameplay consists of fighting various enemies (mostly the various forms of mutants, who prefer to attack in huge swarms) and solving some simple puzzles, usually involving switches.
The game is now freeware and its engine is open source.
The game's source code is available at The Abuse Project.
Fan made levels are available here (archived, some links are broken).
Cheat docs are available here.
Also available for PC.
A Mac OS X (PPC only) version is available here, but it's based on the PC version's game data.
Be sure to select the lower one as the upper is unstable.
Also some universal binary builds are available at The Abuse Project's download page, under attachments.
The latest source release actually contains a mix of the shareware, pc, Bungie/mac & Free Abuse data sets. Currently no official builds of the latest source are available though.