//Steam/HL1/CS/CZ Launch Commands
-noforcemaccel -noforcemspd -noforcemparms -freq 120 -heapsize 1048576 -noipx -nojoy -console
//HL2/Source
+map_background none -novid
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-noforcemaccel		; Use desktop mouse acceleration settings
-noforcemspd		; Use desktop mouse speed settings.
-noforcemparms		; Use desktop mouse buttons settings.
-noaafonts		; Disables Anti-Aliasing of Screen Fonts.
-heapsize 262144	; 512MB System Memory
-heapsize 524288	; 1GB System Memory
-heapsize 1048576	; 2GB System Memory
-w 640 -h 480		; 640x480
-w 800 -h 600		; 800x600
-w 1024 -h 768		; 1024x768
-full 			; Forces the engine to start in fullscreen mode.
-window 		; Forces the engine to start in windowed mode.
-freq 100 		; Changes the Hertz for monitors HL1 Engine. CRT 60-100 85=Common LCD 60-75 72=Common
-refresh 100		; Changes the Hertz for monitors HL2 Engine. CRT 60-100 85=Common LCD 60-75 72=Common
-soft			; This enables Software hardware rendering mode.
-d3d			; This enables Direct3D hardware rendering mode.
-gl 			; This enables Open GL hardware rendering mode.
-nojoy 			; Remove joystick support, more ram free.
-noipx 			; Disables LAN protocall.
-noip			; Some IP related command remove if unable to connect to servers.
-32bpp 			; Enables 32Bit rendering.
-16bpp 			; Enables 16Bit rendering.
-dxlevel 90		; DirectX 9
-dxlevel 81		; DirectX 8.1
-dxlevel 80		; DirectX 8
-dxlevel 70		; DirectX 7
-dxlevel 60		; DirectX 6
-port 27015		; Changes your port # for the game.
-console		; When launching Half-Life, this will take you straight to the console.
-dev			; This is for developer mode.
-zone #			; This allocates more memory to scripting files, like autoexec.cfg and so forth.
-game [mod's name] 	; Enable Half-Life to play modification games other than to go through the menu to select them.
-safe			; It start the game in safe mode disable audio etc.
-autoconfig 		; Restores video and performance settings to default for the current hardware detected.
-condebug		; Logs all console output into the console.log text file.
-nocrashdialog		; Suppresses some 'memory could not be read' unhandled errors.
-novid			; When loading a game with this parameter, the intro Valve video will not play.
-toconsole 		; Starts the engine directly in the console, unless a map is specified with +map.
+a +r_mmx 1		; Instead of using a cfg you can launch the game with a command or a cvar in the command line.
+ex_interp 0.01 	; Sets ex_interp value

Dedicated Server: -applaunch 5
Counter-Strike: -applaunch 10
Team Fortress Classic: -applaunch 20
Day of Defeat: -applaunch 30
Half-Life: -applaunch 70
Half-Life High Definition: -applaunch 96
Condition Zero: -applaunch 80
Condition Zero Deleted Scenes: -applaunch 100
Source Dedicated Server: -applaunch 210
Source SDK: -applaunch 211
Half-Life 2: -applaunch 220
Half-Life 2 Deathmatch: -applaunch 321
Half-Life 2 Lost Coast: -applaunch 340
Counter-Strike: Source: -applaunch 240
Half-Life Source: -applaunch 280
Day of Defeat: Source: -applaunch 300
Darwinia: -applaunch 1500

Server Side:
-nomaster		;Disable client authentification.
-autoupdate		;Enable Auto HLDS Update when the server start.
-insecure		;Disable VAC2.

Credit goes to Solace for compiling all this information :)