Metal Gear

Please also note that if the Abandonware version doesn't have it, you need to create a file called delete.me to get the cheats to work and to skip copy protection.
Ctrl+F1 invincibility
Ctrl+Esc All items

This version of the game is a broken mess
Your gun starts off silenced, collecting the silencer makes it un-silenced.
Terrible hitboxes, losing half your life in one hit and poorly chosen new spawn points mean that you have to use the invincibility cheat to get past the first four screens. Ctrl + F1 to turn on, have to quit and reload to turn off.
Some essential codec messages won't work unless you save the game, quit and come back in. Ctrl+S to save.
You need the Level 8 Card to get into the room where the Level 8 card is, meaning the game is furtherly unfinishable unless you use the cheat to give you all items with Ctrl+Esc. I don't think this game was tested at all beyond the guy who ported it and he just forgot he did that.

Interesting note: Most of the Metal Gear videos on YouTube that claim to show the 1987 version (MSX2, Famicom/NES), in fact are showing the 1990 port for MS-DOS as demonstrated by the copyright year on the title screen.