Paul Brians, professor of English at Washington State University, has published an alphabetical listing of hundreds of common errors in English. Though not everyone will agree with Brians’ interpretations of the language (“The concept of errors is a fuzzy one,” he notes), on balance it’s a very good list. Brians differentiates between commonly misused words, such as “capital” and “capitol,” and also clarifies misused phrases such as “could care less” (“If you could care less,” Brians says, “that means you care some”). There’s also a list of commonly misspelled words and a list of “non-errors,” like ending a sentence with a preposition. Used thoughtfully, it’s a useful site to start a lesson with.