This carries the authority of Bristol University, here in the UK.
It's very easy to use. You just pick whatever figure of speech, punctuation or grammatical issue you want to learn about from the left-hand sidebar. The list is detailed and extensive.
Not only will you get a very thorough explanation of the issue—with clear examples—but then you get to take a 10-question quiz at the end. This gives you a chance to practice what you've learned, and cements the learning process.
I think this might be exactly the resource the forum can use.
Bristol University website 2014-06-03
Very user-friendly grammar and usage resource, with exercises to practice (UK oriented)
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