Paul has compiled a huge list of errors in (American) English usage, listed alphabetically. If you don't know the differences between apiece / a piece, in to / into, and whether / whether or not, his website is worth a visit: Link
An equally fascinating, albeit a little hidden, part of his website is the list of Non-Errors. These are usages people keep on telling you are wrong, but which are actually standard in English. For example:
You shouldn’t pronounce the “e” in “not my forte.”
Some people insist that it’s an error to pronounce the word “forte” in the expression “not my forte” as if French-derived “forte” were the same as the Italian musical term for “loud”: “for-tay.” But the original French expression is pas mon fort, which not only has no “e” on the end to pronounce—it has a silent “t” as well. It’s too bad that when we imported this phrase we mangled it so badly, but it’s too late to do anything about it now. If you go around saying what sounds like ”that’s not my fort,” people won’t understand what you mean.
However, those who use the phrase to mean “not to my taste” (“Wagnerian opera is not my forte”) are definitely mistaken. Your forte is what you’re good at, not just stuff you like.