Immortality?
 

Does anyone know a way to get completely immune to all damage? Or maybe some other cheats??

Yeah, it's called 'Turning the game off.' That way, you can't be hurt!

right...btw graviton..plz resize ur signature it is too big ty.

I do indeed...

right click on your Half-Life icon which I suppose you have on your desktop, where it says Target:, change it to where it looks something like this C:\SIERRA\Half-Life\hl.exe -console
Then open up half-life and there's a new option labeled Console, click on it and type in, sv_cheats 1, then exit the console.
Start a new game and press the ~ key to bring down your console, typing in the following codes will grant you special cheats...
/god = self explanatory
impulse 101 = all weapons and ammo(do this multiple times to fill up on ammo, repeat do refill)
noclip = fly

for other cheats visit Cheating Dome

Imagine if you could get that to work in an online game.
"To avoid that cheaters would download hacks from web sites and eventually get a virus, we decided to include a place in the game where you can type in your cheat codes, so they work online. No more viruses!"