A few more "that makes my teeth itch" examples of common writing mistakes:
1. "Your" vs. "you're"
This is one of the most common mistakes I see, and just about the simplest to avoid if one simply remembers what a contraction is and how to create one. "Your" is a possessive; it always modifies a noun (as in, "That is your sweater, not mine.") "You're" is used correctly only when the meaning is "you are." A correct sentence using both words is "You're going to get your degree soon, aren't you?"
2. "Tenant" vs. "tenet"
Hang around theological discussions much, and you're bound to see this error over and over. D.P. of Disert Paths observed this mistake not too long ago (but I can't find his post at the moment). Just remember that "tenants" are renters who occupy the quarters they rent, and "tenets" are major beliefs within a belief system. It's a stretch to put them both in a single sentence, but here is how they're both used correctly: "The tenants held a meeting to discuss the tenets of their faith."
3. "Loose" vs. "lose"
Here is another case of commonly mistaken identity between two similar-sounding words. "Loose" is an antonym (means the opposite) of "tight." "Lose" is an antonyum of "find." Here both words are used correctly in a sentence: "If your bicycle chain is too loose, you will lose it."
4. "All right" vs. "Alright"
Simply stated, "all right" always is, and "alright" never is. "Alright" is not a word. It is also not a contraction of "all" and "right." Don't use it, please! If you thought it was a word, you're probably confusing it with "already," which is a perfectly good word.
5. "A lot" vs. "alot"
See the rule above. "Alot" is not a word. Use "a lot" instead. Neither should be confused with the word "allot," which means to apportion. The two correct words are included in this sentence: "She did not allot a lot of time for the test." (Say that fast three times.)
6. "Any time" vs. "anytime"
Which of these choices you should use will depend on what you wish to say. "Any time" places the emphasis on "time," while "anytime" is a compound word that refers to an event that is not subject to a specific schedule. For clarity's sake, I won't use them both in the same sentence. This question-and-answer example is correct: "Do you have any time available to meet with me?" "Yes, you're welcome to drop by anytime."
[Stay tuned for the next irregularly-scheduled rant. Some Bat-time, same Bat-channel.]