Synonym.com (find synonyms)

BBC Learning (courses from A1 to C2)

EnglishGrammar.org (online courses, theory, ecercises and tools)

Quizlet (probably the best tool to learn vocabulary online and for smart devices; find my files here for free)

AgendaWeb (online vocabulary and grammar)

Vokker (compile your own vocabulary sets)

English Grammar for You (online grammar)

Englisch-Hilfen (online vocabulary and grammar)

Englisch-Ueben (grammar and idiomatic expressions)

Vokabel.org  (vocabulary trainer)

Langenscheidt Vokabeltrainer (vocabulary according to topics)

Splendid Speaking (vocabulary and grammar training)

IvyPanda (language blog and fun ways to learn vocabulary)