Radio and TV Masts
Radio and TV Masts are some of the world's tallest man made structures. These extremely tall masts are made to support electrical equipment to broadcast radio and television signals. Although they are extremely tall, they cannot be compared to skyscrapers, due to their lack of habitable space. Due to the use of these structures, they are relatively inexpensive, mainly because they are merely steel lattice and cable. The average cost of these masts can be determined by squaring the height of the structure. So a 2000 foot mast would cost approximately 4,000,000 USD. Most of these masts are supported by guy wires (cables).
Mast vs. Tower
A mast in engineering terms is a structure which requires outside support, such as guy wires, while a tower is self-supporting. In both colloquial American and British English, the terms tower and mast are interchangeable. All About Skyscrapers retains the engineering distinction.
Tower

(Seattle Needle)
Mast

(Senior Road TV Tower)
Types of Mast Designs
Parallel-Sided Steel Lattice
The most common structure


Guy Wire Connection

Base Connection
Blaw-Knox

Eiffelized
Towers may be built for the width to taper as height increases. The width grows smaller exponentially compared to the height. This type of structure does not require guy wires. This is referred to as being Eiffelized:

Riga TV Tower

Other mast placements
Other masts can be placed on building tops to minimize costs; examples of these can be seen atop the Sears Tower and Empire State Building.
In some cases, even balloons are flown to support a temporary transmitter. This is often used by the military and amateur radio enthusiasts.
After the modern invention of the cell phone, thousands of towers were constructed to accommodate the huge boom in the cellular industry. These cell towers were often considered ugly and out of place. Today, these towers are being hidden as large trees, or even church towers.