Misspelling

Spelling a word incorrectly. Misspelling is distinguished from other errors in writing, such as grammatical errors, incorrect capitalization or misuse of punctuation. A misspelled word can be a series of letters that represents no correctly-spelt word at all (such as "liek" for "like") or a correct spelling of another word (such as writing "here" when one means "hear", or "now" when one means "know"). Misspellings of the latter type can easily make their way into printed material, because they cannot be caught by computerized spell-checkers.

See List of common misspellings in English.

See List of commonly confused homonyms

And here are some other groups of words for which people commonly write one for the other.

are: plural of is (I am, he/she is, you are, we are, they are)
our: belonging to us

barley: the grain used to make beer (rhymes with "Harley")
barely: hardly (rhymes with "rarely")

breath: the noun (rhymes with "death")
breathe: the verb (rhymes with "seethe")

collage: something made from a variety of magazine cut-outs mounted on paper (rhymes with "garage")
college: university (rhymes with "knowledge")

corpse: dead body (rhymes with "warps")
corps: army or similar organization (rhymes with "four"); also the plural of "corp" when it's short for "corporation"

coup: act of overthrowing a government (rhymes with "new")
coupe: vehicle (rhymes with "group" in U.S. English, pronounced "koop-ay" elsewhere - the word is in fact French, and has an accented 'e' - coup�)

everyday: routine, commonplace; often used instead of:
every day: daily, once per day

now: at the present time (rhymes with "how")
know: be familiar with the facts; be acquainted with; be aware (rhymes with "go")

of: belonging to or somehow connected with; associated with; forming a part of; a certain amount of (rhymes with "love")
off: opposite of on (rhymes with "cough")
've: This is the word "have" as part of a contraction. This sounds like "of" after some words like "could" and "might", but is actually a contraction for "have" (could have, might have). You write: should've, might've, would've, etc.

physics: the laws that govern objects moving in space; related to physical, physiology, physicist, physician (the first syllable sounds like "fizz")
psychic: having ESP; pertaining to the soul; related to psychology, psychiatrist, psyche, psycho, psychedelic, psychopath, psyched (psychic scars) (the first syllable sounds like "sigh")

quite: rather, to an impressive degree (rhymes with "night")
quiet: not very loud (rhymes with "riot")

through: from one end to the other; finished (rhymes with "new")
thorough: complete, exhaustive (rhymes with "burro")

were: past tense of the verb to be (I was, you were) (rhymes with "fur")
where: at what place? (rhymes with "share")
wear: have clothes on; break something down eventually through use (wear out, wear thin, wear and tear) (rhymes with "share")