In addition to their more obvious functions, some moves in MGS2 can be used for other purposes.
Rolling
Both Snake and Raiden can perform a rolling move by pressing X while running. It's useful for quickly evading gunfire (especially shotguns), but in Raiden's case, it can also be used to jump small gaps (notably, the bridge leading to the Shell 2 Core). If you're playing Substance, this tactic also comes in handy on some of the VR Missions.
Hanging
With your back against any railing, press Triangle, and your character will hop over the rail and hang out of sight from enemies. Be careful though - you can't hold on forever. You can also use hanging to drop from some ledges covertly (i.e. without using the stairs), or perform pull-ups by pressing L2 and R2 simultaneously (which will cause your grip meter to drop faster, but 100 pull-ups will increase your strength and allow you to hang on for longer). There are also some points where there is a grabbable ledge right below another - in these cases, you can drop from the upper ledge by pressing X, and quickly press Triangle to grab the next one down. Not only is this handy for getting around, but ten of these will increase your grip strength (much quicker, but far riskier, than doing pull-ups).
Peeking
With your back to the wall, you can use L2 and R2 to peek around corners and get a better view. This will make you slightly more obvious to enemies, but it helps when you need to monitor a troop from a distance. Also, while peeking, you can chuck grenades without exposing yourself (much).
Leaning
While in first-person mode, you can use the L2 and R2 buttons to lean back and forth. In addition, pressing both simultaneously will raise your aiming height, so you can shoot over some dividing walls.
Strategic Cover
While against a wall, and with any standard weapon equipped, stand at the corner and press Square to jump out of cover and aim your weapon as normal. Releasing Square will put you back in cover. Be careful about using First-Person View - it may cancel out the strategic cover, meaning that the game won't put you back around the corner when you've finished shooting.
Lockers
Lockers are a universal stealth tool. Taken at face value, you can opeen them and find all manner of useful items - ammo, weapons, tools, even medicine. Some lockers are locked and you'll have to find a way to open them - beat them into submission with your fists, or place a brick of C4 on them. Keep in mind that the door must be present to be able to hide yourself (or an enemy) in a locker. While inside a locker, you can press the R1 button to bring yourself closer to the vent cover (so you can see better - don't push the button too fast or you'll make a noise), the L2 and R2 buttons to peek around and see things from a different angle, and the X button to crouch out of sight of inquisitive guards (useful if there's a strike team about to look through your viewport at you). Pressing Circle will knock on the locker door and draw attention to yourself. Sometimes you can knock a guard over by opening the door while they're right next to it.