These resources for English language learners are useful are students of many ages and levels. They're all available for free, online.
I recommend EnglishHints for a quick review of most aspects of English, but it's not for beginners, except for a few pages. It's also not organized in a comprehensive way, as the sites in the sections below are.
So these are my suggestions for helpful resources for English language learners of different levels, whether from EnglishHints or other reliable sites, organized by type of practice.
There are several excellent websites that provide an organized, comprehensive set of free English lessons. Two I find especially useful (especially for grammar and vocabulary) are
> English Club (British pronunciations, but the grammar and vocabulary would be almost identical to what's used in the U.S. and the rest of the English-speaking world)
> Learn American English Online (organized by color-coded levels from beginning to advanced English, with different grammar topics addressed at each level)
USALearns also offers several free English courses. They have courses for beginners and intermediates, including one on intermediate reading. Their videos are entertaining but practical, with clear instructions. They also provide many accompanying worksheets and activities.
I'd also like to mention a site that provides a wide variety of ESL games and apps for different ages and needs. It provides simple instructions for many games that can be played without needing a computer (including 20 Questions-- that is explained in more detail here), as well as lots of useful apps. (Note that some of the apps may not be free.)
This website has a lot of different resources that can help English learners prepare for get into, and succeed in grad school. It's focused on getting a master's degree, but many of the resources would be useful for any academic work and very possible for professionals after you've finished university as well.
I recommend starting with the comprehensive list of 25 resources for ESL Graduate Students, but they also have a list of available scholarships, an interview with a former graduate student about her experiences, strategies, and advice, plus more suggestions on how to prepare.
EnglishHints has three sections to learn and practice English grammar. The easiest way to find the specific topic you're interested in is to use the grammar section of EnglishHints' sitemap. There are pages reviewing and/or practicing most aspects of English grammar, from parts of speech to sentence structure, contractions, modal & helping verbs, and different tenses.
EnglishHints also has a very large vocabulary section. Using the sitemap again, the first section lists lesson pages alphabetically. The following section gives practice pages by type of practice, including matching exercises, puzzles, and gap-fills.
The third vocabulary section lists most of the vocabulary pages, (both lessons/explanations and practice) by their subjects, to make them easier to find.
If you're looking for information on the roots and affixes that build English words, see the links on Roots, Prefixes, and Suffixes.
I hope these links will help you find the grammar or vocabulary topic you're looking for on EnglishHints.
For the most useful idioms I would recommend Topcorrect's 50 Common Business Idioms and its linked page on conversational idioms.
The EnglishHints' selection of idioms is limited (though Sports Idioms includes many of the idioms most used in business conversations in English.) Many sites have huge lists of idioms that include less common idioms and are too long to learn. Topcorrect's page is "just right"-- not impossibly long and including many of the idioms most important to understand.
For phrasal verbs, I think the EnglishHints' section is as useful as you will find. If you want to learn to recognize and use them, I highly recommend my free downloadable resource on Common Phrasal Verbs. It's a reference you can refer to again and again, since you're likely to hear many phrasal verbs that aren't easy to figure out on your own.
If you're a beginner or low intermediate English reader, see Easy Reading for ESL Beginners.
My Online Reading page has some good suggestions for intermediates, as does Finding English Reading Materials (for books & other offline sources.) So do a number of Englishhints pages with articles on different subjects. They're linked from Reading Articles to Improve your English.
If you want questions to check how well you understood after reading, see Comprehension Exercises and its linked pages to practice reading comprehension.
Comprehension Exercises also links to sites with good listening materials at several levels. For beginners, see A2 English Listening Practice with TED talks. Actually, these TED talks are so good I recommend them to everyone, including native English speakers.
I especially recommend them for beginners (once you have some basic listening skills in English) because some people familiar with the European system of grading for language difficulty say these talks are A2.
Other TED talks are also great listening and vocabulary practice but are not quite as simple. Official TED talks (& some TEDx) also provide a transcript, so you can check your understanding. Most also offer captions in English and some other languages.
Most TED talks and many other English-language videos can be adjusted for speed. That's valuable if you want to shadow (repeat after each phrase to work on your pronunciation). It's also important if you need to slow a section down to understand it better, or to increase the speed to get used to hearing rapid English.
(I recommend occasionally listening to something you're interested in at 1.2 or even 1.5 speed the first time. to see how much you can understand. Then set it at normal speed and listen again. This is a great way to increase your listening comprehension and confidence. After a few times, normal-speed speech will start to seem easy!)
Incidentally, if you need listening materials in English for children, see the link to 55 ESL resources at the bottom of the Speaking and Writing section below. I just listened to a couple of children's picture books read very expressively (in link #11, Storyline.)
I'm going to recommend it for my (native English-speaking) grandchildren. I also hope to suggest it to friends in other countries who want to give their children a chance to listen to stories in English. The lovely illustrations help make the meaning clear even if the words are mostly unknown.
See Develop Your English Speaking Skills for information on my intermediate-advanced lessons for professionals & other speaking help. Even a few lessons focused on your individual needs can help you speak more confidently and fluently in English.
I also have a section on writing, but the most important resource is my Revision and Proofreading Checklist. I recommend using it or a similar tool before submitting any important writing you do in English to your boss or teacher (or for publication.)
A great alternative is the Virtual Writing Tutor, which will check the form, grammar, punctuation, vocabulary, etc. of writing that you paste there and make suggestions. (It will also check and score essays.) It has other tools as well.
Most universities have free resources for improving or checking your English writing. I highly recommend Purdue's OWL (Online Writing Lab.)
For a very helpful list of sites with useful speaking, writing, and listening help see 55 ESL Resources for Second Language Learners. Several of the links are useful for children, unlike most of the materials on my site or the other writing resources above. I'm familiar with most of the sites mentioned (for kids or adults) and they are excellent.