Firstly to be able to use console commands in left 4 dead you need to enable them. This can be done via the game menu.You need to enable the developer console option then to access the console after the game is started simply press the ~ key which is next to the number 1 key.

thirdpersonshoulder - toggles character view mode (bugged shotgun sounds)
bind <key> "<action>" - binds an action / multiple actions to a key
example: bind x "vocalize smartlook" (very handy in game)
kick <character> - kicks the selected character from the game
example: kick Zoey (getting rid of the bots)
sb_add - fills empty bot slots with fresh bots (spawns kicked bots)
crosshair - toggles crosshair visibility - 1337 mode (boolean 0/1)
net_graph <number> - shows connection, fps and other info overlay in game ( no overlay 0 - full info 4)
changelevel <levelname> - loads selected map (start with "l4d" to get console hint on map name)
sv_search_max_ping <value> - as a Lobby Leader, you can specify the max ping values for server search (default 250)
spectate - moves you to the spectator mode (also handy in VS to change autoswitched players back)
cl_playerspraydisable - toggles player spray texture visibility (boolean 0/1) - thx to ZENTRiERER <hVn>
voice_recordtofile - starts/stops recording microphone inputs to voice_micdata.wav in game dir (boolean 0/1)
startmovie <parameters> - starts recording the gameplay (explore the parameters in console)
example: startmovie testmovie jpg wav jpeg_quality 75
endmovie - stops the recording
record <demoname> - records a demo (.dem file)
stop - (stops demo recording)
playdemo <demoname> - replays a saved demo file
retry - retries connection to the last server used
status - shows user and other information in console (only usable in-game)
team_desired "<team character_name>" - uses the set character preference on server join
example: team_desired "Survivor Bill"

noclip - fly mode
thirdperson - Tomb Raider camera view
god - IDDQD mode (boolean - 0/1)
kill - kill yourself with generic damage
explode - kill yourself with explosive damage (same effect as kill)
z_spawn <character> - spawns a desired character or mob at your cursor
example: z_spawn witch / z_spawn mob
z_common_limit <number> - changes the limit of zombies drawn at one time
give <item> - equips your character with item or effect desired
example1: give molotov / give ammo / give health (anti-knockdown)
example2: give oxygen_tank / give propane_tank / give gas_can
givecurrentammo - refills your current main weapon ammunition
sv_infinite_ammo - current weapon gets infinite ammunition (boolean 0/1)
nb_delete_all - clears all infected, bosses and bots in the area
director_force_panic_event - calls a horde
director_panic_forever - neverending panic event trigger (boolean 0/1)
nb_blind - infected become blind (boolean 0/1)
cl_drawhud - removes HUD elements from screen (boolean 0/1)
boom - spawns an active pipebomb at your feet
fire - spawns an active molotov at your feet
warp_all_survivors_here - instantly ports everyone to cursor position (handy when the bots are a bit away)
sb_takecontrol - you take control of the bot desired
example1: sb_takecontrol Bill
z_tank_has_special_blood - changes tank's blood to a darker sample (boolean 0/1)
z_gun_kick <value> - adds a kick effect to all the guns
sv_crash - if an admin ever screws with you on a sv_cheats 1 server, use this to crash him
achievement_status - checks achievement status through console (handy with Nothing Special, Untouchables etc.)
host_timescale <value> - changes the game flow speed (useful for video editing)
cl_glow_blur_scale <value> - controls the halo size around characters/objects, use 0 to disable the glow entirely