Online Dictionaries

Oxford Advanced Learner’s

Cambridge Advanced Learner’s

Merriam-Webster Advanced Learner’s

Learning English

BBC Learning English

VOA Learning English

Grammar

English as 2nd Language
A comprehensive overview of important grammar points, ranging from beginner to advanced levels.

English Grammar Online
A German website for beginners

The English Grammar Book
Practice your grammar (advanced level)

Vocabulary

The Oxford 3000

Merriam Webster’s 3000 words