New Codec Frequency:
In your codec, have you ever wonder what the last row and last columm for? Try calling frequency 140.07 to talk to the staff. Unfortuately, the conversations are in Japanese, so I don't know what they are talking about. There is no English voice-overs available.

Strange noises:
You can heard some strange noise if you call 140.66 in some area, the one I know is 'The Connecting Bridge'.

Title Screen Tricks:
At the title screen, press any of the directions on the D-Pad to change the background colors.

First-Person View Mode:
The first-person view mode will be available in the special option once you have completed the game once, except with difficulty level, Very Easy. This mode will be quite similar to any Doom or Quake style games. To switch between first person view mode and normal mode, tap triangle twice rapidly. Note that your rank during your first play doesn't affect anything.

Alternate Outfits:
If you complete the game two times with both endings revealed and have stealth and Bandana as your special item, both Solid Snake and Cyborg Ninja will have their alternate outfits, tuxedo and Crimson red and blue cyborg suit when you play the game for the third time. So, what's the difference between the original Metal Gear Solid and the Metal Gear Solid Integral? The difference is, beside Solid Snake, and Cyborg Ninja, Meryl will now have a new outfit, which is like Snake's sneaking suit.