Two/too/to

The correct spelling for the number 2 is "two."  Think "twice" before spelling it out.  There's a "w" in both words, as well as in other variations of the number such as "twenty" and "twins."

When the meaning is "also" or "excessive," the word is spelled "too."  For any usage other than the foregoing, the proper word is "to."  ("I am going to the store.  They are going to the store, too."  "There are too many people to fit in the car.")  One way to remember the distinction is to think of the way it sounds when spoken.  Technically, the words are pronounced the same, but some people emphasize the "oo" sound of the word "too" more than they do with "to," especially when they're speaking quickly (as we all do sometimes!).  Another way to remember is to keep in mind that the word "too," which signifies "in addition" or "in excess," is the one with the extra "o."

Incorrect: "I am going too the store."
Correct: "I am going to the store."

Incorrect: "This is to much money."
Correct: "This is too much money."

Incorrect: "She will be going to."
Correct: "She will be going too."

Have vs. Of

Quite often, people are thrown by the way a phrase is pronounced, and they subsequently carry the confusion into their written text.  I often see incorrect usage of the word "of," as in the following examples: Should of / Would of / Could of / Must of.  There are no such expressions in American English.  In all cases, the second word should be have, not of.  The correct phrases are should have, would have, could have, and must have.

When these phrases are spoken, people will often use the contracted form of the two words, as in: Should've/Would've/Could've/Must've, which doubtless led to the mistaken belief that the second word is "of" rather than "have."  Sometimes in casual speech, folks will use the slang terms "shoulda," "woulda," "coulda" and even "musta," which helps to blur things further, especially given the fact that the slang word "kinda" does mean "kind of."  In informal writing, the proper and correct contractions for Should have/Would have/Could have/Must have are Should've/Would've/Could've/Must've.  In formal writing, it is best to spell out the two words.  In any case, the word "of" is never used.

The foregoing refers to phrases of intent.  For other usages, such as "I must have a cup of coffee now" or "The dog should have a bath," it would be incorrect to write or say "must've" or "should've."

To summarize:

Should of, would of, could of and must of are all incorrect.
The correct phrases are: Should have, would have, could have, and must have.

The contractions should've, would've, could've, and must've are used informally.

Incorrect: "We should of gone to the beach."  "I must of left my bathing suit at home."
Correct: "We should have gone to the beach.  "I must have left my bathing suit at home."  "We could've gone to the beach today."  "We would've gone to the beach if it hadn't rained."

They're/Their/There

Their, there and they're are soundalike words that have entirely different meanings.  Starting with the easiest one to define, the contraction "they're" means "they are" - and that's all it means.  Using anything other than "they're" for "they are" is incorrect, as is using "they're" to mean anything but "they are."  Further, the usage of "they're" for "they are" presumes that one or more words follow the contraction.  Writing or saying "They're late" is correct, but writing/saying "Here they're" for "Here they are" isn't.

As for the other two, the word "there" has multiple meanings, but possession isn't one of them.  Many people write "there" when they mean to use "their," as in belonging to two or more people ("They need to watch their language."  "They're out of their minds.").  Their is a possessive pronoun, just like your, our, his or her.

For anything other than "they are" (they're) or "belonging to them" (their), the correct word is there.  There often refers to a place or destination, and an easy way to remember this is to keep in mind that the words "where" and "here," which also refer to places/destinations, are spelled similarly, and "where" and "there" both have the word "here" in them.  ("There they are!  They're late, and they've forgotten their books.")

Incorrect: "Their out of there minds."
Correct: "They're out of their minds."

Incorrect: "Their they are."
Correct: "There they are."

Incorrect: "They forgot they're books."
Correct: "They forgot their books."

Your vs. You're

"Your" is a possessive pronoun, meaning belonging to you (see also the entry for yours).  An example of proper usage is: "Is that your bicycle?"  When intending to convey "you are," the word that should be used is "you're."  "You're" is the contraction for "you are."  The words "your" and "you're" are often mixed up in informal writing, mainly by using the former in place of the latter (although I've also seen the error made in reverse).

Incorrect: "Your the best."

Correct: "You're the best."

Correct: "You're taking your bicycle with you, aren't you?"

Summary: If you mean "you are," use "you're."  If not, "your" is the spelling that should be used.

It's "its," not "it's" - most of the time

An extremely prevalent word usage error these days is that of "it's," with an apostrophe between the t and the s, when the correct word is "its."  Yes, both are proper words, but "it's" is a contraction of either "it is" (as in, "It's a beautiful day today") or "it has" ("It's been a beautiful day") - and those are its only meanings.  When possession is indicated, the word to use is the possessive pronoun "its," as in the last phrase of the previous sentence.  (This example uses both words: "The painting has lost its luster; it's not as bright as it used to be.")
Of course, I've seen the trend reversed as well, when the apostrophe is incorrectly omitted where it's required.  But that's another blog post.

Possessive pronouns, such as "its," "whose," "hers," "ours," "yours," and "theirs," do not require apostrophes.  Spellcheckers probably won't flag either of the first two, because "it's" and "who's" are proper words (there are, however, no such words as "her's," "our's," "your's," or "their's").  This can be confusing because most other nouns and all proper names do use the apostrophe-"s" combination to indicate posession ("Whose bicycle is that?"  "That is Sally's bicycle.").  But, as we all know, there are exceptions to nearly every rule.  As a consequence, it's up to human beings to memorize the rules of grammar pertaining to the usage of "it's," "its," "who's," and "whose," or to use an on- or offline reference when unsure.  (There are links to a few good online ones under the Resources heading in the sidebar of this blog.)

To summarize:

It's = it is or it has (contraction)
Its = belonging to it (possessive pronoun)
  Incorrect usage: "The snake is shedding it's skin."
  Correct usage:  "The snake is shedding its skin."  "It's going to be a good day."

Who's = who is or who has (contraction)
Whose = belonging to whom (possessive pronoun)
  Incorrect usage: "Who's bicycle is that?"  "There's the man who's eyes are two different colors."
  Correct usage: "Whose bicycle is that?"  "There's the man whose eyes are two different colors."  "Who's going to the store?"

Her's, their's, our's, your's = these words do not exist

See also Common Errors in English Usage - one of the resources listed in my blog's sidebar, and a good one.

It's "a lot," not "alot"

One of the most common spelling errors I've seen over the last decade or so has been the usage of the non-word "alot," in place of the proper two-word phrase "a lot."  A lot - two words, often followed by the word "of"unless it falls at the end of a sentence - means many, or a great deal.  An easy way of remembering this is to mentally use the phrase "a whole lot," which will serve as a reminder that there is a space between the words "a" and "lot."  (Check out Common Errors in English Usage 2nd Edition.)

The confusion about the nonexistent word "alot" may have stemmed from the actual word "allot," which means "to assign as a share" or "to distribute by lot."  (Source: Merriam-Webster.)

Thanks a lot for reading!