Grammar and Writing:
ESL lounge is a great because it has two main areas, one for students and one for teachers. Each of these lounges has content and resources for each target area, for example, on the teacher webpage there are links to lesson plans, blogs, tips from ESL teachers (this is nice if your school doesn't have an ESL department to assist you in teaching ESL students), pedagogy, and assessments, etc. The Student webpage has links at various learning levels for vocabulary, actives, grammar guides, pronunciation, and games, etc.
Virtual writing tutor was created by a college professor who worked with English language learners. It can provide 24/7 support, checking grammar, sentence structures, tenses, and spelling, etc. It also provides explanations of grammar rules. Additionally, there is a page for teachers that offers resources that can be used in and out of the classroom.
This a website dedicated to forums for English language learners, here they can ask native speakers questions. This is good for all ESL students, but especially for those who may be shy or too scared to ask the teacher or a classmate for help.
Students can type in a sentence and hear it pronounced back at them or can record their own voice and hear it played back. This helps them to listen to where they may be making pronunciation errors, or it can be used to show if they have errors in their writing.
This website has tons of resources and lessons for English language learners and a section dedicated to pronunciation.
This is a place where students can create, exchange, and share flashcards of vocabulary words. Students can create their own or look though subjects and find preexisting cards to study from. This is good not only for ESL students but also for native speakers.
This website has lots of actives and games for English learners to play, test and build their vocabulary at various learning levels.
Voxy: This is free but has in app purchases
Babble: This is free but has in app purchases
iTunes store also has an entire section for English language learning in the Education Category.