Midair Collision

So you have a bunch of flying things, usually aircraft, and something's destroying them. How to make it look cool? By having one of the planes damaged enough to cause it to veer off course and crash into another airplane.

Film

In Peter Jackson's remake of King Kong, one of the biplanes smashes into another biplane. Possibly justified as Kong did kind of throw the plane at the other one.
In Transformers, Starscream shoots up a squadron of F-22's, causing one of them to spin of of control and crash into another one.
In Up, a group of dogfighters get distracted by a squirrel (well, not really), and proceed to crash into one another.
- Another Pixar example would be the scene in Cars 2 where Mater and Finn McMissile are both attempting to flee from Grem and Acer at Tokyo Airport, and in the process one of McMissile's projectiles collides with those of the villains.
The World War I film Aces High has a visually impressive full frontal impact between two bi-planes as its climax.
Rinzler actually does this to Clu 2 at the end of Tron: Legacy.
Jumba actually does this to Gantu at the end of Lilo and Stitch with his spaceship (originally a stolen passenger jet) so that Stitch can rescue Lilo.

Live-Action TV

It wasn't shown on screen, but it was invoked by terrorists in 24, using a MacGuffin to crash two planes together.

Video Games

This is one of the most embarrassing ways to fail a mission in the Ace Combat games, especially crashing into a bomber you are chasing or a plane you already shot down.

Western Animation

In Teen Titans there are a bunch of missiles heading toward a bridge so Speedy starts jumping on each of them to change the course so they crash into each other.
Happens near the end of the episode of Quack Pack where Donald Duck is forced to go back into the navy and accidentally tampers with the computer controlling the airplanes on the aircraft carrier.

Real Life

This occasionally happens in real life when multiple RC planes are flying in close proximity, as seen here
Very rarely happens in Real Life, though there were cases in WWII of very tightly packed bomber groups having planes be damaged by debris from other planes being destroyed.