Mute/Moot (Point)
The first (rhymes with “cute”) means [a] unable to speak or vocalize, [b] one who is incapable of speech, or [c] to soften or muffle the sound of. The second (rhymes with “boot”) means [a] without significance, having been previously decided or settled, or [b] of no practical importance, irrelevant. You can set your TV set to mute in order to answer the phone. You can also stop discussion of something already resolved, since it’s a moot point (for example, it’s a moot point whether the chicken or the egg came first).

Of or Have
As in “You should of been there” rather than the correct “You should have been there.” The first one is a preposition and the second one is a verb. Because they sound similar when used in a sentence like that, they’re often used incorrectly.

Its or It’s
The first is an adjective, the possessive form of the pronoun “it.” The second is the contraction for “it is” or “it has.” In usage, you would say “It’s terrible that a polar bear has to die because its habitat is melting due to global warming.” If you can’t substitute “it is” or “it has,” then you should write “its.”

Whose or Who’s
The first is an adjective, the possessive form of the pronouns “who” or “which.” The second is a contraction of “who is” or “who has.” In usage, you would say “He is a man whose power has greatly eroded but who’s still captured the hearts of many.”

Site or Sight
The first refers to a place, a location, a setting, a website. The second refers to the ability to see, or the act of seeing, or something seen or worth seeing, or the foreseeable future. In usage, you would say “The fight over the site of the new shopping mall looks like it has no end in sight.”

Irregardless or Regardless
The second is an adjective that means “unmindful” or “heedless” or an adverb that means “in spite of everything” or “anyway” (for example “This old car continues to run regardless of how old it gets”). The first simply does not exist.

Irrespective or Respective
The first, an adjective, is an archaic version of “regardless” (see above). It also means “characterized by disregard; heedless.” Regardless is the preferred word choice in common usage. The second, another adjective, means “relating to two or more persons or things regarded individually” or “particular.” In usage, you would say “Both Batman and Superman are successful in their respective fields.”

Status Quo or Quid Pro Quo
The first is a noun that means “the existing condition or state of affairs” or simply said, “the way things are now.” The second means “an equal exchange or substitution” and is commonly used in a context of “you scratch my back and I’ll scratch yours” or “you do this for me and I’ll do that for you.”

Condemn or Condone
Both are verbs. The first means to “express strong disapproval of” or “to pronounce judgment against” or “to declare unfit for use,” among similar others. In usage you’d say “This old building was condemned” and “The charitable organizations condemned the needless waste of food and water” and “Ken Lay was condemned to 200 years in prison.” The second is quite different, meaning “to overlook, or disregard (an offense) without protest.” In usage, you’d say “Failure to protest the immigration policy may imply willingness to condone it.”

Effect or Affect
The first can be a noun (meaning “a result”) or a verb, while the second can also be a noun (with the stress on the first syllable, meaning “a feeling or emotion”) or a verb (with the stress on the last syllable). The most confused pair is effect and affect as verbs because they’re similar. Effect as a verb means “to bring about, to bring into existence, to produce a result” and affect means “to have an influence in, to impact something.” Thus the sentence “These measures may affect savings” implies that the measures may reduce savings that have already been realized, whereas “These measures may effect savings” implies that the measures will cause new savings to come about. Both are correct, but their meanings are different and their usage can be tricky.

I Couldn’t Care Less or I Could Care Less
The first means “to be completely indifferent.” In usage, you would say “Wear whatever you like; I couldn’t care less.” Around 1960 people started using “could” instead of “couldn’t” and today both are frequently used. The second one, however, means the opposite of the first.

Tone or Tonality
The second is a noun that means “an arrangement of seven tones built on a tonic key” or “the arrangement of all the tones and chords of a composition.” It also means “the interrelation of the tones in a painting.” The first also has musical and painting-related meanings, but adds the following non-musical ones: “manner of expression in speech or writing.” In usage you would say “The tone of this ad should be upbeat and positive.” The word tonality doesn’t really apply to written or spoken word so you should use the word tone when referring to (for example) tone and manner of advertising.

Mnemonic or Pneumatic
The first means “something that is used as an aid in remembering” (like making up a sentence to remember the planets of the solar system (in order from the sun) in which the first letter of each word is the first letter of the planet’s name: My Very Eager Mother Just Sewed Us Nine Pajamas). The second means “something relating to air or other gasses” (like a pneumatic drill).