To, Too, Two!!!!!!!!!

[edit] Examples of usage
Some strong examples of semantic changes caused by the placement of hyphens:

disease-causing poor nutrition, meaning poor nutrition that causes disease
disease causing poor nutrition, meaning a disease that causes poor nutrition
a man-eating shark is a shark that eats humans
a man eating shark is a man who is eating shark meat
a blue green sea is a contradiction
a blue-green sea is a sea whose colour is somewhere between blue and green
three-hundred-year-old trees are an indeterminate number of trees that are 300 years old.
three hundred-year-old trees are three trees that are 100 years old.
three hundred year-old trees are 300 trees that are 1 year old.
three-hundred-year old trees are an indeterminate number of trees which started to be considered as "old", 300 years ago.
Additional examples of proper use:

text-only document or the document is text-only
Detroit-based organization or the organization is Detroit-based
state-of-the-art product or the product is state-of-the-art (but The state of the art is very advanced. with no hyphen)
board-certified strategy or the strategy is board-certified
thought-provoking argument or the argument is thought-provoking
time-sensitive error or the error is time-sensitive
case-sensitive password or the password is case-sensitive
government-issued photo ID or the photo ID is government-issued (but ?is issued by the government with no hyphen.)
light-gathering surface or the surface is light-gathering
award-winning novel or the novel is award-winning (but, more likely, ?won an award with no hyphen)
web-based encyclopedia or the encyclopedia is web-based
fun-loving person or the person is fun-loving
how to wire-transfer funds
how to tax-plan
advertising-supported service or service is advertising-supported (but, better, ?is supported by advertising with no hyphen.)
Rudolph Giuliani is an Italian-American (but see hyphenated American)
list of China-related topics ?list of topics is China-related (but ?related to China with no hyphen)
out-of-body experience
near-death experience
in surnames, for example Dominique Strauss-Kahn
Note, though, that many authoritative sources, such as the Chicago Manual of Style, recommend writing commonplace compounds open (i.e., without hyphen) when they appear after the noun they modify and when they are used adverbially. Thus

She always wears out-of-date clothes.
but

Her wardrobe is out of date.
Similarly, for the adverbial use compare

The hand-to-hand combat was frightful.
and

They fought hand to hand in repulsing the attack.