Grammar can be evil. There I said it. Chastise me if you want.
Everyone knows that the Book of Style is your first reference for grammar rules; however, often we need additional information or would simply like to explore certain grammar rules more in depth. Do you know what an interjection or a squinting modifier is? There are a lot of grammar sites, but I do like this one for their examples and clarity. I really learned a lot from the Modifiers section. I actually found it because I use their service to grammar check my papers.
What grammar sites do you like?