Online dictionary of collocations that also has an app version. Very useful for finding out not only what words mean, but also seeing how they are used.
Site with grammar explanations and practice exercises. Great for advanced and C1 learners who would like to review the use of adverbials, verb phrases, etc.
Sometimes listening to audio tapes of books is an effective way to develop fluency and create reading/listening opportunities while on the go. Lit2Go is a free online collection of stories and poems in Mp3 (audiobook) format. An abstract, citation, playing time, and word count are given for each of the passages. Many of the passages also have a related reading strategy identified. Each reading passage can also be downloaded as a PDF and printed for use as a read-along or as supplemental reading material.
Free online courses in English and other languages, with pronunciation and practice activities. Has app for Apple and Android, making it very useful for self-study.