-- Can/may. These two aren't interchangeable. Use "can" when you are denoting ability, as in, "Do you think I can win the race?" Use "may" when you want to establish permission: "May I borrow your book?"
-- Compare with/compare to. Write "compare with" when you're making a literal comparison. "Our profit this quarter was up when compared with our profit in the same quarter last year." Use "compare to" to make a figurative comparison. "Laura compared assembling the annual report to running a marathon."
-- Since/because. "Since" is the proper word when you want to express a period of time, as in, "It has been years since I visited there." Use "because" to give a reason or cause. "Production is slumping because our machinery is old and broken." The common error involving these two is to misuse "since," as in, "I refuse to sign this report since I disagree with it." That's wrong. Swap "since" with "because."
-- Principal/principle. I recently discovered an error with this pair on a colleague's Web site. "Principal" is someone who is first in rank or authority, as in the principal of a high school or the principal speaker at a convention. "Principle" refers to a truth, doctrine or rule of conduct.
-- Eager/anxious. Writers frequently use "anxious" when what they really mean is "eager." Use "anxious" when you're describing a state of fear and worry. Think "anxious" and "anxiety." "Eager" is the word when you want to indicate a mood of stimulation or excitement.