Arcanum can be started with the following arguments:

-fullscreen
Play the game in full screen (compact UI) mode.
-fps
Display current FPS in the top left corner.
-nosound
Play the game without sound.
-norandom
No random encounters.
-scrollfps:[number]
Scrolling speed. If not specified it will be the default value of 35.
-scrolldist:[number]
Scrolling distance. Set it to 0 to make it infinite. Default: 10.
-mod:[name of a module]
Runs a different module at startup.

-no3d
Play the game in safe mode (software renderer).
-vidfreed[number]
Increases art cache by [number]
-3dref
Uses D3D REF device instead of HAL. (D3D features with software acceleration, very slow, used for testing).
-doublebuffer
Double buffer. May increase performance.

-nonmsmousez
Standard mouse wheel behaviour. (a redundant option?)
-msmousez
Alternative mouse wheel behaviour.

-mpautojoin
Turns on AutoJoin for Multiplayer.
-mpnobcast
Turns on NoBroadcast for Multiplayer.

Command line arguments related to debugging:

(-)tigdebug=[console/debugger/file/mono]
Overrides registry settings. E.g. -tigdebug=console (sets console to 0), tigdebug=debugger (sets debugger to 1), etc.

console - shows debug output in a separate window.
debugger - outputs debug info to your debugger.
file - prints debug info to debug.txt.
mono - outputs debug info to monochrome monitor memory.
-dialognumber
Shows PC dialogue line numbers in game.
-dialogcheck
Checks all dialogue files for missing lines, prints the results in the debug output.
-gendercheck
Checks if characters have correct gender specific descriptions when loading a map. Prints all errors in the debug output.
-logcheck
Checks all log entries, prints the results in the debug output.

Following arguments have been cut out:

-animdebug
-patchlvl

The effects of these arguments are still unknown:

-animcatchup
-pathlimit (probably -pathlimit[number])
-pathtimelimit (probably -pathtimelimit[number])

If you find out what any of these command line arguments do, please tell me and I'll update this thread.

Thanks Drog for finding out what most of these arguments do.

I've found out what almost all of them do.

-fps // Shows an fps counter in the top left corner.
-nosound // No sound.
-no3d // Software renderer. No severe memory leaks in this mode.
-3dref // Uses D3D REF device instead of HAL. (D3D features with software acceleration, very slow, used for testing).
-doublebuffer // Double buffer. May increase perfomance.
-vidfreed[number] // Increases art cache by [number].
-animcatchup
-animdebug // CUT OUT
-norandom // No random encounters.
-nonmsmousez // Alternative mouse wheel behavior.
-msmousez // Standard mouse wheel behavior. (a redundant option?)
-pathlimit
-pathtimelimit
-mpautojoin // Turns on AutoJoin for Multiplayer.
-mpnobcast // Turns on NoBroadcast for Multiplayer.
-fullscreen // Compact-UI mode.
-patchlvl // CUT OUT
-dialogcheck // Checks all dialog files for missing lines, prints the results in the debug output.
-dialognumber // Shows line numbers next to all PC lines.
-gendercheck // Checks if characters have correct gender specific descriptions when loading a map. Prints all errors in the debug output.
-scrollfps:[number] // Scrolling speed. Default: 35.
-scrolldist:[number] // Scrolling distance. Set it to 0 to make it infinite. Default: 10.
-mod:[name of a module] // Runs a different module at startup.
-logcheck // Checks all log entries, prints the results in the debug output.
(-)tigdebug=[console/debugger/file/mono] // Overrides registry settings. E.g. -tigdebug=console (sets console to 0), tigdebug=debugger (sets debugger to 1), etc

console - shows debug output in a separate window.
debugger - outputs debug info to your debugger.
file - prints debug info to debug.txt.
mono - outputs debug info to monochrome monitor memory.

Thank you, I've updated the first post.