You're probably familiar with Google Docs, the free, Web-based word processor, spreadsheet, presentation, form, and data storage service offered by Google. But you may not be aware that the Template Gallery contains a number of 'gadgets' which are really useful vocabulary building tools. For example,the Flash Cards spreadsheet gadget allows you to create flash card sets easily and quickly.
Other useful gadgets include Word Search and Word Study. Word Study even lets you use hints from Google Images:
COMMENT You might want to check the results before using Word Study with students. These are the pictures automatically provided by Google Images as a hint for the word 'fox'!