Not sure where to put this, so I think the Tutorials is a good place.
-safe(Run the game in safe mode) -a(Unknown) -s(Unknown, maybe same as -safe) -basedir(Sets the base directory of the mod, example: valve) -num_edicts(Unknown) -condebug(Enables console debugging, writes to qconsole.txt even when condebug disabled from console) -demoedit(Enables Demo Edit mode) -noforcemparms(Don't force mouse parameters) -noforcemspd(Don't force mouse speed) -noforcemaccel(Don't force mouse acceleration) -steam(Launch game as it was on-Steam) -gl_log(Unknown, maybe logs OpenGL commands if the game is running in OpenGL) -gl11(Forces the game to launch in OpenGL 1.1) -ad(Loads .ad file, still unknown purpose) -0.001(Unknown) -console(Enables console on game startup) -toconsole(Same as -console) -dev(Enables Developer mode) -voicerecord(Records microphone voice on startup) -clockwindow(Unknown) -freq(Set the frequency rate) -port(Set the port number on which deticated server will run, default 27015) -noip(Don't use localhost IP Address) -noipx(Don't use IPX) -reuse(Re-use the network packet) -netsleep(Unknown) -netthread(Unknown) -loopback(Unknown) -particles(Unknown) -nosound(Disables sound entirely in the game) -simsound(Simulates sound in the game) -snoforceformat(Unknown) -primarysound(Use primary device for sound, still unknown purpose) -wavonly(Allow WAV files only for sound) -maxplayers(Set Max Players value for deticated server) -insecure(Launch the game in in-secure mode) -nomaster(Disable Master, still unknown purpose) -starttime(Unknown) -surfcachesize(Set value for Surface Cache, still unknown purpose) -heapsize(Set heap size for how much memory will be used) -minmemory(Set heap size for how much minimal memory will be used) -noaff(Unknown) -noaafonts(Disables anti-aliased fonts) -16bpp(Launch game in 16 bitmap image color) -24bpp(Launch game in 24 bitmap image color) -32bpp(Launch game in 32 bitmap image color) -window(Start game in window mode) -windowed(Same as -window) -startwindowed(Same as -window) -sw(Launch game in Software Mode) -full(Launch game in Fullscreen Mode) -fullscreen(Same as -full) -d3d(Launch game in Direct3D Mode) -gl(Launch game in OpenGL Mode) -dll(Selects the DLL file from mod directory which should be used for API, still unknown purpose) -w(Set width of game screen) -h(Set height of game screen) -width(Same as -w) -height(Same as -h) -forceres(Forces resolution for windowed mode, some versions support this) -novid(Disabled game startup video) -zone(Used for Memory_Init value, specify the value in KB, E.g. 256.000)
Update - Added new parameters
-debugbreak(Unknown) -nocrashdialog(Disables game crash dialog) -noassert(Disable game asserts) -mpi_worker(Unknown) -assertsteam(Unknown) -dibonly(Unknown) -stdvid(Unknown, but probably doing something with video driver) -sport(Unknown) -nofbo(Unknown) -gldrv(Takes a specific OpenGL driver from .\gldrv folder) -nosteam(Launch game as it was non-Steam) -tcp(Unknown) -udp(Unknown) -lognetapi(Unknown) -single_core(Runs game in Single Core CPU mode, might not work on modern systems) -gameoverlayinject(Forces Steam Overlay by injecting the needed libraries in game) -from_mce(Unknown) -noservice(Runs game in Steam without Steam Client Service) -debug_steamapi(Tries to debug Steam API while game is running) -s3(Unknown) -mousethread(Sets how much CPU threads should be used in order to smooth out the mouse movement) -silent(Launches the game silently, some versions support this) -conheight(Determines how much big in height should console be for HLDS) -usegh(Unknown) -novgui(Launches HLDS without Valve Graphical User Interface) -highpriority(Launches HLDS in High Priority) -maxfps(Sets Maximal FPS for HLDS) -RANDOMIZEMEMORY(Randomize memory blocks) -NOINITMEMORY(Disables Memory_Init block, still unknown purpose) -ccsyntax(Unknown) -all_languages(Enables All Languages support on Valve Graphical User Interface 2)