Cheat: Console Mode
Even a single player game like Half-Life feels like it has more cheats than your average PC title. Many of these can be accessed through the game's Console Mode.
To get Console Mode, right click on the Half Life desktop shortcut. In Properties, go to the Target tab. Look for the line that reads: X:\Sierra\Half-Life\hl.exe. Put "-console" at the end of that line so it reads: X:\Sierra\Half-Life\hl.exe -console. Note the space after ".exe". Now launch the game and notice that the startup screen has a "Console" option. There are a ton of cheats you can enter, so here is a large collection of them.
If you just want Invincibility, enter: /god in the Console Mode.
If you want Invisibility, enter: notarget in the Console Mode.