I still recommend advanced students look at books like R W Burchfield, The New Fowler’s Modern English Usage (3rd edition Oxford University Press 1996), but the best these days is The Cambridge Guide to English Usage ed. Pam Peters (2004). The text is not online but the site does feature an online quiz.
There are several good books on usage aimed at ESL students. Practical English Usage by Michael Swan (OUP 1995) is something of a classic. There are some sample pages on that site.
For easier explanations see this very useful book: A Student’s A to Z of Grammar, Usage and Style by Steve Moline (Melbourne, Oxford University Press 2002.) This book is very much up-to-date and tells you a lot about types of writing, problems in grammar and word-choice, media study, computing terms… Excellent from Year 7 all the way to Year 12.
Sites? Well, there are a few, and if you look at the links page under English Language you will find some.