Is it one word, two words, hyphenated or en-dashed? Here's a quick reference.

A

able-bodied
aboveboard
absent-minded
ad-lib
A–frame (Uses an en dash.)
air bag
air base
air-condition
air-conditioned
air conditioner
air conditioning
air force (Capitalize when referring to the U.S. Air Force.)
airline
airmail
airman
airtight
airways
all right (NEVER alright)
also-ran
altogether (Altogether, they made up 30 percent of the work force.)
all together (They were all together when they died.)
any time (NEVER anytime)
anything
around-the-clock
artwork
audiovisual
autoworkers (common noun)
Auto Workers (part of the name of a union, a proper noun)
awe-struck

B

baby-sit
baby-sitting
baby-sat
baby sitter
bookstore
backyard (adjective: It was a backyard barbecue.)
ball carrier
ballclub
ballpark
ballplayer
ballroom
ball point pen
barrel-chested
BB gun
best-seller
big time (noun: We've hit the big time.)
big-time (adjective: We've made a big-time mistake.)
bimonthly
biweekly
blackout
brownout
blast off (verb: The rocket is ready to blast off.)
blastoff (noun and adjective)
bloodhound
blue blood (noun: She has blue blood.)
blue-blooded (adjective: It was a blue-blooded gathering.)
bonbon
boo-boo
bowlegged
boyfriend
breast-feeding
bullfight
bullpen
bull's eye

C

cakewalk
camouflage
carefree
caretaker
carmaker
carpool (noun, verb and adjective): one word in all uses, in contrast to AP and Webster's; He belongs to a carpool; The carpool lanes were closed; He carpools to work; also vanpool
carry-over (noun and adjective)
caseload
chain saw
cease-fire (noun)
cell phone (noun or adjective)
cha-cha (use a hyphen)
chalkboard
changeover
check up (verb: I'm going to check up on her.)
checkup (noun: It's time for your annual checkup.)
child care (two words in all uses)
churchgoer
citywide
clean up (verb: We need to clean up our copy.)
cleanup (noun: It will be a Superfund cleanup.)
close-up (noun and adjective)
coastline
coattails
colorblind
cookbook
commander in chief (no hyphens; capitalize as a title before a name)
cornstarch
countryside
courthouse
courtroom
cover up (verb: Mr. Burruss told her to cover up those knees.)
cover-up (noun: It was a cover-up. adjective: It was a cover-up scheme.)
crack up (verb: I hope I don't crack up.)
crackup (noun: There was an awful crackup on the highway.)
crisscross
cross-examine
cross-examination
cross fire
crossover
cross section (noun: It was a cross section of voters.)
cutoff [date, for example]

D

dark horse
data base (AP style, an exception to Webster's)
day care (two words in all uses)
daylily (exception to Webster's because of common usage)
daylong
daytime
deathbed
deep-sea (adjective: It was a deep-sea dive. noun: It was an awfully deep sea.)
deep water (noun)
deep-water (adjective)
doublespeak
double standard
download (noun or verb)
drop off (verb)
drop-off [site, for example] (adjective)
drop out (verb)
dropout (noun)
drugstore

E

easygoing
editor in chief (no hyphens; capitalize as a title before a name)
empty-handed
en masse
en route
evenhanded
even-steven (Steven is lowercase here)
every time (NOT everytime)
ex- (As a prefix meaning "former," use a hyphen: her ex-husband)
eyestrain
eyewitness

F

fact-finding (adjective)
fallout (noun)
far-flung
farsighted
fast-food (adjective)
ferryboat
filet (noun) Only refers to the specific dish filet mignon
fillet (noun) Refers to any general cut of meat
firearms
firsthand
flagpole
flashcard
flimflam
flip-flop
floodgate
flood plain
flood tide
floodwall
floodwaters
folk song
follow up (verb)
follow-up (noun and adjective)
fortuneteller
fortunetelling
freewheeling
front-runner
full time (adverb: She works full time.)
full-time (adjective: He has a full-time job downtown.)
fundraising (noun: Fundraising is difficult.)
fundraising (adjective: They planned a fundraising campaign.)
fundraiser (noun: They held a fundraiser. They hired a fundraiser.)

G

game plan
getaway (noun and adjective)
get-together
girlfriend
go-between
good will (noun: It was a gesture of good will.)
goodwill (adjective: It was a goodwill gesture.)
grade-point average (GPA on second reference)
grass roots (noun: They are the grass roots of my campaign.)
grass-roots (adjective: It was a grass-roots organization.)
groundswell
groundwater (an exception to Webster's because of common usage)
grown-up (noun and adjective)
gunbattle
gunfight
gunfire
gunpoint
gunpowder

H

hair spray
hairstyle
hairstyling
hairstylist
halftime
handmade
hand-picked
hard-core (adjective)
harebrained
headlong
head-on
health care (two words in all uses)
heartwarming
hit-and-run
hitchhike
home-grown
homemade
home page
hometown
hot dog (the wiener)
hot-dog (the showoff)
hot line
hourlong

I

inasmuch as

J

jack-o'-lantern
JellľO (uses an en dash)
jukebox
jump-start

K

kewpie doll (uppercase K only for the actual trademarked doll; lowercase for the imitations given as prizes at fairs, etc.)
kickboxing
knickknack
know-how
kowtow

L

lamebrain
lame duck (noun: He was a lame duck.)
lame-duck (adjective: It was a lame-duck administration.)
lead (verb: You can lead a horse to water, but you can't make it drink.)
led (verb, past tense of lead: He led the horse to water, but it wouldn't drink.)
left hand (noun)
left hander (noun)
left-hand (adjective)
left-handed (adjective)
leftover (noun and adjective)
left wing (noun: He was proud to be part of the left wing.)
left-wing (adjective: He was a left-wing candidate.)
left-winger (noun: He was a real left-winger.)
letter-writer
lifelong
life-size
lifestyle
lighthearted
long shot (noun: It was a long shot.)
long-shot (adjective: She was a long-shot choice.)
long-standing
long term (noun)
long-term (adjective)
long time (noun: It took a long time.)
longtime (adjective: They were longtime friends.)
lowercase (AP style, an exception to Webster's)

M

make up (verb)
makeup (noun and adjective)
man-made
mantel (noun: The mantel over the fireplace is covered in soot.)
marketbasket
marketplace
medium-sized
middle-aged
middleman
mix up (verb)
mix-up (noun and adjective)
mind-boggling
mock-up
moneymaker
monthlong
mo-ped
multi (No hyphen unless it precedes a capital letter. Note placement of the hyphen in the common construction multimillion-dollar budget.)
multimillionaire

N

na´ve
narrow-minded
nationwide
nearsighted
nighttime
nitpicking
no man's land
no one
nowadays

O

oceangoing
oddsmaker
officeholder
off-season (noun and adjective)
off-site (adjective and adverb)
Olympic-sized
online
on-site (adjective and adverb)
onstage
op–ed
outargue
outfox
outpatient

P

paddy wagon
pant-suit
pari-mutuel
parkland
passer-by, passers-by (AP style and an exception to Webster's)
peacekeeping
peacemaker
peacemaking
peace offering
pigeonhole
pile up (verb)
pileup (noun)
pingpong
pin stripe
place mat
playlist
point-blank
policy-maker
policy-making
pom-pom (a rapid-firing automatic weapon)
pompom (the thing cheerleaders wave; never pom-pom or pompon, according to AP style and an exception to Webster's)
pothole
potluck
power plant
postseason
preseason
push-button
push up (verb)
push-up (noun and adjective)

Q

quick-witted
quizmaster

R

red-haired
redhead
redheaded
red-handed
red-hot
redshirt
rib cage
riffraff
right wing (see entries for “left wing”)
round-the-clock
runoff (noun)

S

school-age (NOTschool-aged.)
seat belt
second hand (noun: The clock had a second hand.)
secondhand (adjective: It was a secondhand coat.)
semiannual
semiautomatic
showoff
sign up (verb)
sign-up (noun or adjective)
single-handedly
soft-spoken
sometime
sometimes
spinoff
stand-up (adjective: He was a stand-up comedian.)
stormwater (an exception to Webster's because of common usage)
sugarcoat

T

tae kwon do
teen-ager
tepee
theatergoer
tiebreaker
touch-screen
townhouse (an exception to Webster's because of common usage)
T–shirt
trade-off
trick-or-treat, trick-or-treating
tryouts
twentysomething

U

undercover (adjective)
under way
upfront

V

video game
videotape
videocassette recorder
voice mail

W

wisecrack
work force
workhorse
workplace
winemaking

Y

yardstick
yearlong
year-round

Z