Macmillan Dictionary

Macmillan Dictionary is free and it doesn't require any subscripition in order to be fully used. In terms of resources available, we can access "Language Resources", "Pronunciation" (through listening and phonetic transcription), "Clear Definitions", "Red Words and Stars", "Thesaurus", "New Words", "Real Examples" among others. It should be highlighed the fact that it in "Real Examples" section, it provides rather clear explanations on how to use a word in a very specific context of grammar structure, eg the collocation of an adverb in a sentence.

Oxford dictionary

Although the first impact isn't very actractive, it doesn't require any subscription, it offers free english resources, including puzzles, games, word of the day, which can only be photocopied. Unfortunately, the listening practise is not available. In a specific section "Better writing", there is a wide range of grammar, spelling, punctuation, practical writing, improving english and abreviattion tasks. It's only available the English language dictionary. The activity we chose was Headword hunt, which turned out to be very boring.

Cambridge dictionary online

Cambridge Dictionary is totally free, it has got five types of dictionaries ("Advanced Learner's", "Learner's", "American English", "Idioms" and "Phrasal Verbs"), translation, how to pronounciate (both British and American pronounciation), word/image ("picture of the day"), semantic, network, phonetic transcription and thematic general words.
It also offers a wide variety of grammar exercises to be done online, including the final scores and solutions. The list of resources is available from the home page. These resources are focused on listening, reading and writing.
After having gone through an exercise for elementary level, aimed at students aged 12-14, having in mind to test the students' ability dealing with elementary grammar structures, we came to the conclusion that, without being a boring task, it's rather useful for students to practise on dealing with the same topic in several different ways.