Start the game with the command line parameter hl.exe -dev -toconsole . Now while playing you can press ~ to bring up the console.

It is also reported you can go to the console by pressing ~ then type in sv_cheats 1 instead of having to do the command line parameter above

Invincibility /god
Go to Map (insert map name for name) /map name
Get Item (Insert item name for item) /give item
Weapons & Ammo impulse 101
No Clipping & Flight /noclip
Enemies ignore you /notarget
Third Person view /thirdperson
Give Weapon /give weapon ###
Gravity change Use numbers 10-800 for ## sv_gavity ##