Like the title of this post says, Ninja Words is a super fast online dictionary. Just type a word in the search box and Ninja Words almost instantly provides the definition(s). Ninja Words uses Wiktionary to provide definitions. You can get single definitions or type in a combination of words get and compare multiple definitions. If you're using Firefox or IE 7+ you can use Ninja Words in your search toolbar.
Thanks to Twitter user Ozge for the tip about Ninja Words.
Applications for Education
Ninja Words is a handy little resource for your students to access on their computers. You may want to link it to your classroom blog so that students can quickly look up words that they don't know while reading blog entries.
Here are some related items that may be of interest to you:
Wordnik Has Everything About Words
Leximo - The World's Multilingual Dictionary
Math Dictionary For Kids - Visual Dictionary