Ghost in the Shell: Stand Alone Complex

Unlock extra weapons:
When an enemy drops a weapon that you don't have in your inventory,
pick it up, and make sure to finish the level while still holding it.
When you start a new level the weapon will be permanently added to
your inventory.

Unlock Tachikoma minigame and Movie Viewer:
Just beat the game with any character in order to unlock a SpyHunter
like minigame featuring Tachikoma, and a Movie Viewer that let's you
watch all of the game's cutscenes again.

Unlock Various Dual Handguns:
If you start a mission with only a handgun equipped, at the end of the
mission you will be rewarded with that same handgun in its dual form - 2
of the same gun that can be equipped at once, only needing 1 weapon slot
to hold. You can use this method to unlock a dual version of each of
the standard handguns in the game.

 