A list of tools like this is a great way to get started exploring what tools are available for education. Of course, the real benefit is gained when other teachers share their experience and ideas.
Visual vocabulary tool. Shows definitions, parts of speech, aynonyms, antonyms and "fuzzynyms." Fuzzynyms are closely related ideas/words that aren't true synonyms. Good for visual learners
This wiki is a starting point for exploration. It fosters connections between teachers and tries to highlight Web 2.0 tools used in classrooms around the world.