Grammar
English Grammar Today is another option on the extensive Cambridge Dictionary website. It functions via a search box as well as an A-Z browsing function - useful if you don't know the exact term for what you want! British English only.
There is also a useful Grammar Guide from Oxford Dictionaries - the interface is slightly less crowded than the Cambridge version. British English only. (Note that the explanations are less detailed than in English Grammar Today.)
Collins Easy Learning English Grammar is a print resource that is also freely accessible online. (Just look for the table of contents on the left. It begins with parts of speech.) The resources are available as (paid) e-books as well.
Grammaring.com* - start with the list of topics.
*What sort of site is this? To quote: "Grammaring.com is a shortened version of the e-book The Grammaring Guide to English Grammar. While it does not contain timeline diagrams, quotes, and exercises, which come with the e-book version, it has some extra features that make it a practical resource for intermediate to advanced students of English as a second or foreign language."
The Internet Grammar of English is an online course in English grammar written primarily for (British) university undergraduates - it is very helpful if you'd like more information from a linguistics perspective.
Vocabulary and Expression
Academic Phrasebank. This database, created by John Morley at the University of Manchester, has an extensive collection of useful phrases for academic speaking and writing. There is also a version that can be purchased and downloaded for off-line use.
The Academic Word List and the Oxford Phrasal Academic Lexicon are both useful word collections linked to dictionary entries.
SKELL - Sketch Engine (language corpora software) for language learning. The simplifed student interface has a lot of interesting options for exploring and describing authentic language use. (If you don't really know how to use a corpus, then this is a good place to start.)