Koopa Shell

Use in Mario platformers

Koopa shells have been around almost as long as Mario himself. First appearing in the original Mario Bros., they always serve one purpose: to be kicked.
Koopa shells are usually obtained by jumping on Koopas, causing them to hide inside their shells. Then you kick them away, wiping out any enemies that happen to be in the line of fire.

As the series went on, Koopa Shells gained new uses and powers. For example, in Super Mario World for the SNES, different coloued shells will give a different effect when Yoshi eats them. Blue shells makes him grow wings and fly, red causes fire breath, yellow grants him a ground stomp power, and a flashing shells does all three. When Mario entered the 3rd dimension, koopa shells could be used as a make-shift skateboard, used to skim across lava and access new areas.
The most recent addition to the shell's joblist arrived in New Super Mario Bros. on the DS, where a blue shell could be found in boxes. When used, it gives Mario the ability to shoot across the floor, smashing blocks in his way. Many secret areas were uncovered using this item.

Use in the Mario Kart series

Koopa shells have appeared as weapons in the Mario Kart series since its debut on the SNES. They currently come in 4 different flavours, all of which have different behaviours and powers.

Green Shell

One of the most common weapons in the series, the Green shell is fired straight forwards, rebounding off walls and barriers before eventually smashing. 
Can also be found in groups of threes which rotate around the player's kart, creating a shield until all 3 are used. 
When hit with a green shell, the player's kart will spin to a halt.
 
 
 

Red Shell

Has the same effect as the Green Shell, only this one will heat-seek towards the opponent, and instantly smash if it hits a barrier. Can also come in clusters of 3.

 
 
 
 
 

Blue Shell

One of the rarest (pre- Mario Kart Wii) and most powerful weapons in the game, the universally-loathed blue shell is feared and hated by all. 
This shell will fly above the track and head directily towards the player in 1st place, finally slamming into the ground, causing a large explosion. 
It is literally impossible to dodge, and always seems to hit you at the worst time: right before the finish line.  The Blue Shell has been specifically criticized as a particularly unfair element of the game, punishing skilled players and rewarding luck.

 

Bowser's Shell

This heavy shell is modelled after Bowser's and is Bowser's and Bowser Jr.'s special item in Mario Kart: Double Dash!! When thrown, it will crush anyone in its path and make them lose all items currently in possession.