As an editor, I’m always looking for ways to improve my grammar. The grammar posts here often help me as much as (or more than) they help my readers, since they force me to consider the reasons why a grammar rule is in place.
I’ve been a fan of Grammar Girl for years; her explanations are helpful and clear, and she’s my go-to resource when I’m not quite certain about something specific.
Another wonderful resource is GrammarBook.com. There are concise explanations and examples of just about any rule you could want to learn – and quizzes to reinforce them! There are plenty of free quizzes, but if you want even more you can subscribe for a nominal fee (currently it’s under $30 for everything or $0.99 for individual quizzes).
Where do you find answers to questions about grammar?