If you do a lot of writing or editing, I recommend keeping a copy of either the Oxford Style Manual, or the Chicago Manual of Style to hand. These are comprehensive reference books, used my two major publishers. The former focuses on British English, whilst the latter American English. Pick one or the other depending upon your likely audience. They are amazing for just dipping in to with a specific problem; they have excellent indexes.
The Washington State University website has a good, alphabetical section on common errors in English.
This page on WikiHow is a six-part series on how to use English punctuation.
Six short notes on the usage of commas, apostrophes, quotations marks.