There are numerous sites with vocabulary games and dictionary/thesaurus facilities on the WWW: as an ESL teacher you should familiarize yourself with some of these sites. Once you have selected the site, you can connect your computer to a large screen and the whole class will then be able to participate in the activity: make sure that the activities you select are appropriate for the CEFR level of the class you are teaching. A few sites that you might consider using are the following (they have been chosen at random, and no recommendation is implied – although they do seem to be useful for ESL learning):
http://www.vocabulary.co.il/syllables/ Vocabulary.coil: this site has a large selection of games and language activities for all ages.
http://quizlet.com/ Quizlet is a flashcard (another useful tool) learning websites: you can actually make your own flashcards for a variety of subjects.
http://www.snappywords.com/ Snappywords is an interactive dictionary and thesaurus: it also visually shows the connection between words.
http://www.wordhippo.com/ Wordhippo functions, among other things, as a dictionary of synonyms and antonyms.
It may be that your students ask you to recommend a good dictionary, in this case – any of the following are well suited for ESL learning:
Merriam Webster: http://www.learnersdictionary.com/ (This is a US English dictionary.)
Another useful tool that the ESL teacher can use to build up the student’s vocabulary is the regular use of a spelling test combined with a cloze test of, say, twenty words a week: this will help the student learn the spelling, pronunciation and meaning of the words: over a twenty-eight week period, a guaranteed 560 words would be learnt every academic year – and this is apart from any additional vocabulary that the student might learn during the course of the academic year.
At the end of the day, ‘old fashioned’ reading is still the best method for improving vocabulary: to this end the ESL teacher can use a graded reader combined with a student’s personal dictionary as tools for building vocabulary: the personal dictionary would be used for writing the meanings of all new vocabulary encountered in the reader.