Bomb (Links)

Bomb is Samus, Link, Young Link, and Toon Link's Down Special.

Link's Bombs
The bombs of the various versions of Link are items only they posseses.

The bomb can be thrown at other opponents, where it will explode on impact. The character hit by Link's bomb will take about 6% damage, Toon Link 5-9%. If the bomb is dropped, then other characters can pick it up and use it as an item. The bomb can also be caught by other characters. When the bomb is flashing red, that indicates that it is about to explode. If a bomb explodes while a character is holding it, the person holding it and people in the immediate vicinity then they will take reduced damage, around 3%. It is interesting to note that while Link's bombs deal damage to breakable terrain such as the towers of Shadow Moses Island, and the floors of the Skyworld, Toon Link's do not. As the bomb is an item, they cannot use the move if they is already holding another item.77

Young Link's bombs are exactly the same as Link's and Toon Link's, but they hit multiple times. They also have a slightly different design

The bombs from SSB and SSBM use graphics from Ocarina of Time. In SSBB, Link's bombs are from Twilight Princess and Toon Link's from Wind Waker.

Bomb Spiking
Bombs can also be used to setup Link's spike. When a bomb is thrown on a airborne opponent under him, the opponent will be stunned, allowing for Link to spike them.

Origins
The bomb has appeared in almost all Zelda games, including the original The Legend of Zelda. However, the bombs have been largely reduced in size. Link holds his bombs in one hand in the Super Smash Bros series, while in his respective games, he holds them with both hands over his head, since the bombs where much larger. The bombs flashing red when it is about to explode also comes for the Zelda series. The bomb design is based on the Link's respective game he appears in.

Bomb Smashing
If the A button is held during an animation where Link/Young Link/Toon Link cannot throw his bomb (such as during a roll, or during the time when he takes out the bomb), it will allow the player to do a forwards smash with the c-stick without throwing the bomb. The bomb can then be thrown after the smash, stunning the opponent and setting the opponent up for a dash attack, slide smash, or a dash attack cancel.

Bomb Braking
In Brawl, if a Bomb explodes while in knockback, it increases the knockback Link and Toon Link travel. However, if Link or Toon Link use their zair right before it explodes while in knockback (other than the knockback of the Bomb), it cancels all knockback allowing them to survive at percents they'd usually get KO'd for that move. However, it is very situational because it takes 7 seconds for a Bomb to explode and the attack that knocks them has a chance of making them dropping the Bomb. Without Link or Toon Link having a way to force the bomb to explode, there's rarely a chance of pulling it off with a result of surviving longer.

Video showing the Bomb Braking

Samus' bombs
Bomb is Samus's Down Special Move in all three installations of Super Smash Bros. series.

Samus shifts into morph ball mode and lays a bomb, which will explode either after a set time period or upon contact with a character, shield, or breakable hitbox. If Samus is on top of a bomb when it explodes, she will shift into morph ball mode and bounce up; thus, if Samus uses Bomb while touching an opponent, she will hit the opponent or their shield with the bomb and be popped up into the air. The mechanics behind the Morph Ball result in the Super Wavedash in Melee. Bombs almost always hit twice, although the two hits are essentially indistinguishable; if a bomb drops onto an opponent from above, it may hit them once and bounce off without exploding. Bombs may do between 2% and 7% damage in SSBM.

Bombing while in mid-air causes Samus to go into Morph Ball mode, and slows down her fall. Thus, in Super Smash Bros., Samus can repeatedly bomb while holding towards the stage to better her recovery, although this takes away her midair jump if she still has not used it. In Melee, bombing twice consecutively in the air will cause Samus to pop upward from the first bomb, giving her near-infinite horizontal recovery. "Bomb jumping", as this is known, is reputedly possible in Brawl, but is less efficient.

Bombs are a large part of Samus's approach in SSBM. Simply running, wavedashing, or SWDing towards the opponent and bombing off of them gives several options. If the bomb hits, a fast-fallen aerial will almost certainly hit as well during the stun, and can lead to easy combos or edgeguards. If the bomb is shielded, Samus can either fast-fall an aerial anyway and go into grounded shield pressure with jabs, tilts, and grabs; bomb again while still on top of the opponent and severely wear down the shield, or simply jump away onto a nearby platform. Wavebombing, a technique found by Co_and_me, consists of Bombing off an opponent followed immediately by a downwards airdodge: this functions as a quick wavedash and allows various continuations.

In SSBB, however, Samus is capable of also using her bombs to perform the Spider Ball technique. To perform, run off the edge of the stage or platform and immediately activate the bomb, then direct Samus back onto solid ground as she hops into the air. This will drop the bomb directly against the ledge of the stage, creating an effective edgeguard in itself as well as a setup for other possible attacks, such as a Down tilt or Down aerial spike.

Origin
Morph Ball bombs have been standard in all Metroid games, along with the Morph Ball Bomb Jump, a common tool for puzzle solving. The appearance of the Morph Ball in all three Smash Bros. games is derived from the original Metroid, due to its lack of spiral patterns. In Brawl, it is also more detailed. Also, in a number of the Metroid games is a more powerful version of the Bomb, aptly called the Power Bomb. It is always obtained very late in the games, and often the last power-up; this is due to it being essentially the most powerful weapon in all of Samus' arsonal, as it is makes a massive explosion capable of penetrating virtually every type of armor/shielding other than Samus' and incinerates all living things in it's range as well as obliterating almost everything else. (Of course, in games this is limited to eliminating/damaging enemies and only destroying extremely select obstacles, despite it being effectively a near-spammable small nuke.) It's destructive capabilities as compared to the Zero Laser are unknown, though most likely the Power Bomb is still magnitudes more powerful.

Trivia

 * In Melee, Link and Young Link's bombs will explode if two hit while in midair. In Brawl, Link and Toon Link's bombs pass through each other if they meet in midair.
 * In Event 3: Bomb-fest, in Super Smash Bros. Melee, in which Link & Samus are opponents, they almost always uses their Bombs to attack.