Mario Kart: Double Dash!! introduced many new elements to the Mario Kart series, but right now, let's talk about special items.
Special items are items exclusive to certain characters (and King Boo + Petey Piranha, who are allowed to use every special item). Though this concept is present in Mario Kart Tour, Double Dash was the first entry to use it. Each pair of characters on the selection screen (Mario + Luigi or Yoshi + Birdo, for example) shares a special item. These are:
Fireballs - Exclusive to Mario and Luigi. Luigi gets green fireballs because he's special. Launches a spread of fireballs in front of your kart. They bounce a few times before disappearing and ricochet against walls. Opponents will spin out if hit.
Heart - Exclusive to Peach and Daisy. Protects you from and allows you to steal up to two items, with no time limit. The only items that can negate the Heart are the Spiny Shell, Lightning, Star, Chain Chomp, and Bob-Omb.
Yoshi/Birdo Egg - Exclusive to Yoshi and Birdo. Birdo gets pink spots on her egg because she's special. Homes in on the kart in front of you and, either upon landing a hit or running out of time, will break open to scatter three items on the track. Possible items include Bananas, Green Shells, Mushrooms, Fake Item Boxes, Stars, and Bob-Ombs.
Chain Chomp - Exclusive to Baby Mario and Baby Luigi. Summons a Chain Chomp to pull your kart forward at high speeds. The Chain Chomp will eventually break off and run wild across the track for a few seconds. It will knock racers over and force them to drop their items if it hits them, regardless of whether it is still attached to you.
Giant Banana - Exclusive to Donkey Kong and Diddy Kong. Just like an ordinary Banana, but giant and angry. If another kart hits it, it will split apart into three regular Bananas. Can also block other items (such as Shells) without breaking or disappearing.
Triple Shells - Exclusive to Koopa Troopa and Koopa Paratroopa. Just like normal Green and Red Shells, but you get three. You will, however, drop all but one if you get hit by an item while holding them. I have heard that Paratroopa is more likely to draw Red Shells, but I cannot verify this.
Bowser Shell - Exclusive to Bowser and Bowser Jr. Launches forward and can ricochet against several walls before breaking. It will knock racers over and force them to drop their items if it hits them.
Bob-Omb - Exclusive to Wario and Waluigi, and making its overall series debut in this entry. It explodes by hitting a racer, colliding with an item, or reaching a time limit. At the very outer edge of the explosion, karts only spin out. Toward the center, though, racers are tossed into the air and forced to drop their items.
Golden Mushroom - Exclusive to Toad and Toadette. Provides an unlimited number of speed boosts within a time limit.
So, based on these descriptions (and your own gameplay experience, if applicable)...
Which special item is your favorite?
Fireballs
Heart
Yoshi/Birdo Egg
Chain Chomp
Giant Banana
Triple Shells
Bowser Shell
Bob-Omb
Golden Mushroom
Call me King Boo/Petey Piranha, because I want them ALL!