Affect vs Effect - Affect (verb: influence), Effect (noun: result)
Lie vs Lay - Lie (recline, no object), Lay (place, needs object)
Rise vs Raise - Rise (go up on own), Raise (lift something)
Accept vs Except - Accept (receive), Except (excluding)
Lose vs Loose - Lose (misplace), Loose (not tight)
Then vs Than - Then (time/next), Than (comparison)
Your vs You're - Your (possessive), You're (you are)
Its vs It's - Its (possessive), It's (it is / it has)
Advice vs Advise - Advice (noun), Advise (verb)
Practice vs Practise - Practice (noun, US also verb), Practise (verb, UK)
Principal vs Principle - Principal (head of school), Principle (rule/belief)
Stationary vs Stationery - Stationary (not moving), Stationery (paper products)
Desert vs Dessert - Desert (dry area), Dessert (sweet course)
Complement vs Compliment - Complement (completes), Compliment (praise)
Elicit vs Illicit - Elicit (draw out), Illicit (illegal)
Emigrate vs Immigrate - Emigrate (leave a country), Immigrate (enter a country)
Ensure vs Insure - Ensure (make sure), Insure (insurance)
Farther vs Further - Farther (physical distance), Further (abstract or additional)
Fewer vs Less - Fewer (countable), Less (uncountable)
Who vs Whom - Who (subject), Whom (object)