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.
Each other: not eachother.
Each other’s: other is singular in this phrase, so it is made possessive by adding ‘s.
Effete: (1) lacking vitality, (2) decadent, or (3) effeminate. The last sense is born from long misuse of the word in place of effeminate.
Elephant in the room: a large, obvious, and important thing that no one wants to address.
Eleventh hour: the latest possible time before something is to happen.
Embed vs. imbed: They are different spellings of the same word, though embed is far more common.
Emphasise vs. emphasize: U.S. and Canada: emphasize, emphasized, emphasizing, etc. Outside North America: emphasise, emphasised, emphasising, etc.
En masse: French for in mass; in English, in one large group. It’s not on masse, in masse, or on mass.
Encase vs. incase: There is no difference between them. Encase prevails by a wide margin.
Enclosed please find: often overformal.
Energise vs. energized: North America: energize, energized, energizing, etc. Outside North America: energise, energised, energising, etc.
Entrap: Interchangeable with trap in senses unrelated to law enforcement and illegal acts.
Entree: The French word from which the English word is derived has an aigu accent over the second e, but the accent is usually dropped in English.
Enure vs. inure: Enure is a less common variant of inure.
Envy vs. jealousy: Envy: a feeling of discontent tied to the possessions or accomplishments of another. Jealousy: (1) fearful of losing something, or (2) covetous of what another has.
Eschew: to avoid, escape, or shun.
Even keel: Something that has an even keel or is on an even keel is steady and predictable.
Evidence vs. evince: To evince is to show or demonstrate something. Evince is not closely related to evidence, but it does work wherever you might be tempted to use evidence as a verb.
Excess vs. excessive: Excess usually applies to measurable things, and excessive usually applies to actions. These are not rules, though, and the words are often interchangeable.
Excrete vs. secrete: Excrete: to eject bodily waste. Secrete: (1) to synthesize and release a substance; (2) to conceal in a hiding place.
Ex officio: by right of office.
Expiration date vs. expiry: In North America, the date at which a product expires is the expiration date. In the U.K., it’s the expiry or the expiry date. Expiry is not used in American English, while expiration is used in British English for numerous purposes unrelated to product expiry dates.
Extol vs. extoll: Extol (meaning to praise highly) is the preferred spelling everywhere, but the preferred inflections are extolled and extolling.
Extraordinaire: extraordinary in a particular field or activity. It’s a rare postpositive adjective—meaning it always comes after the noun it modifies.