Down tilt

A Strong Down, also known as a Down tilt (abbreviated "d-tilt,") is a Strong Attack performed by any character by holding the control stick lightly down or crouching and pressing A while on the ground. It will instantly hit all surrounding characters, making it useful for spacing. It is also useful for a foe rolling around you. It can also be performed by pressing Z while crouching.

In Super Smash Bros. Brawl, many Strong Down attacks have the ability to make opponents prat fall/trip occasionally.

Down tilts in smash 64
Pikachu: A moderately high knockback move with surprising range (similar to most of Pikachu's moves in this way). Less comboable that Pikachu's other moves. Semi-spike.

Ness: Very quick with low damage. Usually used into a throw or against a wall for a shield break. Can be Smash DI'd out of to prevent a grab after.

Yoshi: Another set knockback tilt (the other one being up tilt) with good power, can infinite against a wall, usually used for gimps. Very good for edgeguarding Fox, Jigglypuff and Kirby. High power semi-spike by the ledge.

Falcon: A good move that has IASA frames. Usually used as a shield stunner in order to throw after. Good knockback. Essentially Yoshi's up tilt but without set knockback, slightly less range, and more comboabiltiy.

DK: One of the few low lag DK moves. Essentially a weaker version of Falcon's. Very useful at close range to lead to a grab or down b.

Mario: A low knockback move with a good amount of damage. A sweeping kick. Not overly useful due to poor range and poor combo ability.

Luigi: Similar to Mario but with slightly less range, slightly less lag, and slightly less knockback. Combos into Up B.

Jigglypuff: Similar to Pikachu but slightly less versatile, quicker, and has less range. Good combo ability and works great into rest. A staple Jigglypuff combo is down air, Up tilt x 3 (or more varying on character) and then a Rest.

Samus: A longer range version of Mario. Good knockback and a good spacing move.

Link: A different form of down tilt. Sends an opponent up. Combos into up B and other moves. Too high lag to be used often though.

Fox: Similar to Link's but with more range and slightly more comboability. If there wasn't Smash DI in this game, then this move leads into an infinite with Down air after.

Kirby: A moderate range move that is quite good at gimping and breaking shields.

Character's Down Tilts in Brawl
Mario: Swivels his leg around himself in a circle. Moderate knockback.

Luigi: Different from Mario's (in all 3 games). While crouching, slides foot farther from foe into them, usually causing them to trip. Can be a weak meteor smash if used on a foe on the ledge.

Peach: Bends forward and slaps at a slightly downward angle. If opponent hits her forearm, it can meteor smash off the edge. Has a 30% chance of tripping if opponent hits her hand.

Bowser: Swipes his claws along the ground twice, with high knockback for a down tilt. As Giga Bowser, both swipes are meteor smashes and they are imbued with shadow power.

Donkey Kong: Slaps along the ground with good range. Great racking up damage at low percentages and a trip can be combined into a Hand Slap.

Diddy Kong: Ducks and claps his hands together, hand has a colored "booming" effect. Horizontal knockback.

Yoshi: Swings tail along ground. Weaker at tip.

Wario: Points finger at the ground. Quick, but weak.

Link: Swipes sword very close to the ground. Difficult, but can meteor smash off the ledge.

Zelda: Curves foot forward. Weak but can combo into itself or pop enemy slightly above her. Meteor smashes foes on the ledge.

Sheik: Spins around in a circle while crouching. Similar to Zero Suit Samus' and Sonic's.

Ganondorf: Sticks foot out. Great knockback, but weak compared to Ganon's other moves.

Toon Link: Ground-shaving quick swipe. Good for edgeguarding; provides medium knockback.

Samus: Unique move. Small explosion from her arm cannon is shot at the ground. Very strong, good finisher, no lag, and decent damage.

Zero Suit Samus: Similar to Sheik's and Sonic's. Makes slashing sound and sends foe upwards.

Pit: Crosses bow blades on across the ground. Foe is hit upwards. If used off ledge, it will meteor smash.

Ice Climbers: Similar to Dash attack. Knockback a bit higher than Diddy's.

R.O.B.: Shoots arms very quickly diagonally down at ground. No lag at all, can be spammed, and causes prat falls. edgeguarding spammable. Not much knockback, but good damage racker. Can infinite against walls.

Kirby: Kicks foot forward. Weak, but has a high chance of forcing a trip.

Meta Knight: Pokes sword forward. Similar to Marth's, although faster, and moves him forward a bit.

King Dedede: While crouching, swings foot forward. Ending lag, but strong attack.

Olimar: Pushes forward along the ground. (Comedically, if Pikmin are behind him, it looks like his Pikmin shoved him on the ground.)

Fox: Swings tail along the ground. Low priority.

Falco: Same as Fox', but much more priority due to upwards knockback for setting up a spike combo for edgeguarding. Less range than Fox's, but good for combo setups.

Wolf: Kicks out with his foot, having decent range and can cause tripping.

Pikachu: Spins around once. Like his down smash, but without the electricity.

Squirtle: Spins around and around forward. Multiple hits, good priority.

Ivysaur: Whips its vine low and forward.

Charizard: Reaches out and bites along the ground. Strongest at the tip of the move where his mouth is biting, weaker at his neck area.

Lucario: Pokes foot out and holds arms behind him. Very similar in appearance to Ness'. Aura imbued.

Jigglypuff: Jams foot forward and low. Bad knockback.

Marth: Stabs sword forward. Same as Meta Knight's. Slower than MK's, but still moves Marth forward, like Meta Knight.

Ike: Swings sword heavily horizontally. Extremely powerful ledge spike and always spike aerial opponents very powerfully.

Ness: Is an infinite foot-jabbing combo in SSBM. In SSBB, it causes prat falls. Good hitstun, but less knockback. Like Lucario's in appearance. This move is great setup for Ness' Up Smash if the enemy trips. 100% of tripping if sweetspotted.

Lucas; A non PSI-powered leg swivel. Can be indefinitely used like Ness', but is incapable of going as fast, and does not trap the opponent. Has a 100% chance of tripping if sweetspotted.

Mr. Game & Watch: Flips a 3D manhole in front of him. Excellent move with good knockback and priotity. Good for combo setups.

Snake: Spins body around in a circle, very close to the ground. Has KO potential on most characters at 120% and up.

Sonic: Swings leg around and comes back in same position. Similar to Zero Suit Samus' and Sheik's. Moves forward very far.

In Melee

 * Mr. Game and Watch's and Falco's down tilts, well known for their speed and power. Each of their down tilts sends the enemy upwards and can kill as early as 100%.
 * Marth, Roy, and Yoshi's down tilts, which are low trajectory "pokes" useful for edge-guarding.
 * Ness's down tilt is an infinite on certain stages against fast fallers.

In Brawl

 * Meta Knight's down tilt comes out on frame 3 and has IASA frames on frame 15; this combined with its exceptional range makes it a potent frame trap.
 * Ness's downtilt is extremely fast, and can rack up damage extremely fast. It has a 100% chance of tripping if sweetspotted.
 * Marth's down tilt is fast, very easy to sweetspot compared to his other attacks, and can be used to easily reach over an edge.