Timber!

A Stock Phrase used after a tree or other tall object is felled. Common in cartoons where the tree falls on a character or causes considerable damage to something, or both.

Another gag is to have characters wonder who this Tim(othy) Ber person is, and why everybody's calling for him.

Examples of Timber! include:

Comic Books

Happens in a Teen Titans comic where Mammoth is knocked out. For a few moments, he just stands there looking dazed but then he starts to fall (landing on Gizmo). Changeling calls out "Timber!" as he falls.
Often used in a Canada, Eh? setting, as in Daltons in the Blizzard: the Daltons are wandering through the forest, when suddenly they hear "Timber!" and huddle around trying to understand what it means. One gets as far as "I think it's something lumberjacks yell whenever they're cutting down a-" before being interrupted by the tree nearly falling on them.

Film

Breakfast at Tiffany's: Holly shouts it as a woman in a party passes out.

Literature

In The Steerswoman series, one of the hints about the Outskirters' origins is that, although they live (and believe they have always lived) in a place with no trees, they shout "Timber" when felling a tall object.

Music

Used in metaphorical forest of "Timber" by FaLnX:

But when I take my chainsaw -- Get out of my waaaaay! Timbeeeeer!

Video Games

In Punch-Out!!, lumberjack Bear Hugger says "Timbeeeer!" when knocked down.
In Tales of Monkey Island Chapter 2: The Siege of Spinner Cay, with help from Trenchfoot, Guybrush manages to push the rubber tree down to the ground while shouting out, "Timmmm-boing?"

Web Comics

Schlock Mercenary did a large-scale version here (the pokeball used to collect the "tree" is about 8 kilometers in diameter).

"Timber" is the lumberjack equivalent of "fire in the hole".

Western Animation

Chuck Jones produced an internet cartoon series about a timberwolf who had a tree fall on him everytime he said "Timber".
A typical Tex Avery gag: a character chops down a tree, usually so that it falls on another character. He runs out of the way and starts shouting "Tim..." when the tree falls on him. "..ber."