There is a simply great book by Strunk and White called, "The Elements Of Style"- it's a small book but is so wonderful. I don't know of any particular sites, but I do know that this book helped me a great deal. There's another book I like about this subject and it is called, "Eat, Shoots, & Leaves" - these are both super books about general grammar usage and punctuation.
I was also going to mention Strunk and White's, "The Elements of Style". I also like "Woe is I".
Strunk and White's just the answer I was going to give. No fair you two. No in all seriousness "The Elements of Style" is very thin, almost like a pamphlet. It's only like 85 pages long and it's easy to read. I've read it 4 times myself and could probably use to read it a few more for that matter. When I asked this same question I was told to memorize it. Haven't done that yet.