Note: This image 
shows an item can be thrown both ways.
Green Shell
The green shell is projection weapon that shoots from the front of the kart. When fired from behind, it remains stationary, acting much like a banana peel.
Red Shell
Similar to the green shell except it homes in on the racer in front of you.
Star
Grants the player temporary invincibility and also spins anyone who touches you.
Banana Peel
Drops a banana peel for opponents to slip on.
Mushroom
Gives the player a speed boost..
Feather
Allows the player to jump in a 360 degree spin. Very good for jumping walls to make shortcuts, and also for jumping red shells. Only available in Battle Mode or Match Race and on certain GP tracks (such as Ghost Valley 1).
Ghost
Makes your kart invisible to other players and will steal whatever item your opponent is holding. Only available in Battle Mode or Match Race.
Coins
Gives the player 2 coins.
Lightning
Shrinks all the other opponents. When shrunk, the enemy moves slower and can be run over.
Poison Mushroom
These can only be used by Toad and Princess Peach when controlled by the computer. They can be drop or fired in the same way as banana peels, but the effect of driving over one is very different. Instead of getting a boost, the driver who hits it will shrink and drive slower for a long time until the effect wears off. If the driver is already small, however, a second poison mushroom will restore him or her to normal size.
Yoshi Egg
These can only be used by Yoshi when he is controlled by the computer. He drops the eggs behind him and they stay on the track like a banana peel or poison mushroom. The effect of hitting one is the same as being hit by a shell.
Lava Ball
These can only be used by Bowser when he is controlled by the computer. He shoots a lava ball across the track that spins out someone if they hit it.