Writing is one of the four main skills when it comes to learning a language (the others being Listening, Speaking, and Reading). However, writing is often overlooked in teaching and learning, so here I am going to list some resources which will help you to develop your writing skills.
Academic Writing Basics — A great collection of PDF worksheets from eslflow.com.
CAE Writing Phrases — Exhaustive list of phrases for advanced writing.
Daily Writing Tips — Every day a new article, with topics ranging from grammar to punctuation, from spelling to usage and vocabulary. For advanced learners.
ESL Writing — A super collection of writing activities and guides.
Fraze.it — Improve your writing skills by learning from professional writers. With more than 100 million sentences, this is the most advanced platform for sentences on the net.
Lang-8 — A new language learning platform where native speakers correct what you write.
LearnEnglish Teens — Practise and improve your writing skills for your school studies and your English exams. Activities for different levels.
OWL — Purdue Online Writing Lab — An impressive collection of resources for general and academic writing. Also has a section on job search writing.
Penzu — Private, 100% customizable online journal. Also available as an app.
TelescopicText.org — Create expanded texts. Great fun but read the tutorial first.
Write & Improve — Submit your written work and receive feedback in seconds, covering spelling, vocabulary, grammar and general style.
WritingDEN — Let WritingDEN's Tips-O-Matic help you write better documents.
Writing Skills — English writing skills for a variety of purposes, including essays, formal and informal letter writing, resumes, business documents.
5 Ways to Improve Your English Writing — Useful tips from the RealLife social English learning site.
Academic Help — Database of free guides on writing different types of academic papers.
WC Quick Guides — Downloadable guides from George Mason University Writing Center.
Writing Tips and Advice — These articles from ThoughtCo are aimed at native speakers, but could also be useful for advanced ESL learners.