CONCORDANCERS
This collection compiled by the University of Leeds, provides you easy access to several corpora (collections of texts) including the British National Corpus, British newspapers and others. It is easy to use and does not require registration.
Corpus of Contemporary American English (COCA)
The largest publicly available corpus of English, available since 2008. It has a database of 400 million words including newspapers, fiction, spoken language and academic writing. You can also search surrounding words to see if a particular word is typically used together with other words (collocation). After using it a couple of times you will be prompted to register, however, registration is free.
GRAMMAR AND PUNCTUATION
Extended Rules for Using Commas - Purdue Online Writing Lab, Purdue University
A clear and well structrued overview on the use of commas and punctuation.
Use of Articles - Walden University
Well written explanation and a video on the use of articles.
VOCABULARY
Provided by Manchester University, this resource contains lots of typical phrases you can use in different parts of your papers (without plagiarising). Of course, you can also adapt the phrases you find here to make them your own.
A collection of eight dictionaries and numerous word finding tools, including legal dictionary, computer dictionary, synonyms, related words, homophones and lots more.
Another wide-ranging collection of specialist dictionaries.
This rather interesting resource used to be free - now you can still have a free trial all the time under hard sell.