Researching Web sites about grammar I came across Common Errors in English Usage site. It contains a huge list of commonly misused or misspelled words, organized in alphabetical order. The list is a blast to peruse; although I was already familiar with most of the examples, Brians clarifies subtle points I might not have thought of or known about (e.g. the discussion of e.g. and i.e. was helpful to me).
While you're there, check out the Non-Errors page as well.