Note: Many of the entries here will eventually become full-length posts. Some are rough and have not been fully researched. If you have any corrections or would like to add anything, please comment.
Calumny: A false statement meant to hurt someone’s reputation. Plural calumnies.
Camaraderie vs. comradeship: no difference in meaning.
Can of worms: an issue that when broached is likely to raise a complex set of unresolvable issues.
Candor vs. candour: candor in American English; candour everywhere else.
Cannon fodder: military personnel who are expendable in battle or who are considered likely to be killed.
Cannot: never can not.
Cannot see the forest for the trees: so focused on details that the big picture is forgotten or ignored.
Canoeing: not canoing.
Car park: two words.
Caretaker: one word, no hyphen.
Carrot and stick: There are two ways to motivate a mule. One is to give it a reward—the carrot—for good behavior. The other is to punish it—the stick—for bad behavior. In expressions involving carrots and sticks, this is what they refer to.
Categorise vs. categorize: In the U.S. and Canada, it’s categorize, categorized, catagorizing, categorization, etc. Outside North America, it’s categorise, categorised, catagorising, categorisation, etc.
Cause celebre: a famous cause or an incident that draws great public attention. In French, celebre has two accents—célèbre—but English is not kind to such marks, and we usually omit them.
Cease and desist: Unless you’re in the legal profession, go with one or the other, or just use stop.
Celebrant vs. celebrator: Celebrant originally referred to a participant in a religious ceremony, but today it is used interchangeably with celebrator. If you want to please very careful readers, keep the words separate.
Censer vs. censor vs. censure vs. sensor: A censer is a vessel in which incense is burned. To censor is to remove objectionable material from something. To censure (not a homophone with the others—the s is pronounced sh) is to express harsh disapproval. A sensor is something that senses.
Center around vs. center on: A center is a point, not an area, so center on is more logical.
Ceremonial vs. ceremonious: no substantive difference. Ceremonial is more common, and increasingly so.
Certifiable: fit to be declared mentally ill.
Chalk up: to attribute to a specified cause. Chock up is a misspelling for this sense, though it does have a meaning of its own—i.e., to wedge wooden blocks beneath wheels to prevent them from moving.
Chateaus vs. chateaux: Chateau ( a French manor house,  an estate where wine is produced,  a large country house) is usually pluralized in the French manner—chateaux—but there is nothing wrong with the English plural, chateaus.
Chav: British slang for a youth exhibiting certain stereotypical qualities (which are unclear to us as Americans—we welcome your comments).
Cheek to cheek: Hyphenate it if it comes before what it modifies (e.g., There were a dozen cheek-to-cheek couples on the dance floor). Don’t hyphenate it if it follows what it modifies (e.g., The couples were dancing cheek to cheek).
Chestnut: an old, frequently repeated joke or story. It’s often modified by old.
Chip on one’s shoulder: a habitually negative, combative, or hostile attitude.
Chips vs. fries: What Americans call fries (or French fries) are usually called chips in the U.K., Ireland, Australia, and New Zealand (though some thinly cut chips are called fries). What Americans call chips (or potato chips) are called crisps elsewhere.
Chock-a-block: crammed full, usually followed by with. Nautical in origin. Usually hyphenated instead of one word, though chockablock does appear occasionally.
Choose, chose, chosen: choose is present tense, chose is past tense, and chosen is the participle.
Civic vs. civil: Civic means of or relating to the city. Civil means of or relating to citizenship.
Civilian: a person who is not a member of the military. The word is often stretched beyond logic to apply to people who are outside a certain group.
Civilise vs. civilize: American and Canadian English: civilize, civilized, civilizing, civilization, etc. Outside North America: civilise, civilised, civilising, civilisation, etc.
Claptrap: pretentious, meaningless language.
Cleaned: not cleant.
Cleave: The past tense is cleaved or clove. The past participle is cleaved, cloven, or cleft.
Clockwise: one word.
Cloth vs. clothe: Clothe is a verb meaning to put clothes on. Cloth is only a noun. It denotes a piece of fabric or a fabric material.
Co-ordinate vs. coordinate: The unhyphenated form is preferred in American English. The hyphenated form is preferred everywhere else.
Collectable vs. collectible: Collectible is the preferred spelling.
Collocate: to group or place together. It doesn’t require a hyphen, nor do its derivatives—including collocated, collocating, collocation.
Colonise vs. colonize: colonize in the U.S.; colonise everywhere else.
Comeuppance: what one deserves for his or her past actions.
Coming of age: It is hyphenated when it is a phrasal adjective preceding what it modifies—e.g., a coming-of-age story. It is not hyphenated when it is not a phrasal adjective—e.g., the story is about coming of age. A coming-of-age story is one centered on someone growing up or reaching maturity.
Commoner: For common to a greater degree, more common is more common than commoner, which is primarily a slightly pejorative noun for a person who is not of the nobility.
Compare to vs. compare with: When you measure one thing against another, you compare it to the other thing. When you measure two things against each other, you compare them with each other. The distinction is sometimes so subtle as to be unimportant.
Competence vs. competency: There is no substantive difference between them. Competence is more common.
Complex prepositions: prepositions consisting of two or more words—e.g., according to, apart from, prior to.
Conceded vs. conceited: Conceded is the past tense of concede, which means to to yield or to acknowledge. Conceited is an adjective meaning holding a high opinion of oneself.
Conformance vs. conformity: They share the sense action or behavior in accordance with laws or standards, but conformance is more often used in legal and official contexts. Conformity is more often in other contexts, especially with regard to behavior in accordance with social norms.
Congruent vs. congruous: There is no substantive difference between them. Congruent is more common.
Consistant: a common misspelling of consistent.
Consultative: not consultive (yes, even though it’s longer).
Contranym: a word with a homograph that is an antonym. For example, two definitions of fast are (1) moving quickly, and (2) staying in place.
Converse, inverse, reverse: As nouns, they are interchangeable.
Coon’s age: offensive U.S. slang for a long time. Don’t use it.
Copacetic: very satisfactory. Copasetic is a less common alternative spelling.
Cornflour and cornflower: Both are real things, but they are not related. Both are one word.
Corroborate: to strengthen with additional supporting evidence or testimony.
Creature comforts: material things that provide bodily comfort.
Credible vs. creditable: Credible means believable or trustworthy. Creditable means deserving of credit.
Cottage industry: a loosely organized industry mostly carried out in homes or in small places of businesses.
Crises: plural of crisis. Crisises appears occasionally, but it doesn’t gain ground because it is so hard to say.
Crowdsource: a new word originating in the last few years of the 20th century. It means to rely on a large group of people to collectively provide answers or solve problems.
Cumulative adjectives: successive adjectives that modify their noun in different ways and hence are not separated by commas—e.g., scared and little in the scared little boy.
Curricula vs. curriculums: The Latin plural (curricula) is more common, but there is nothing wrong with the English plural (curriculums). It’s a matter of preference.