This is a great site for revising vocabulary (at a higher level) and helping to donate food to the UN World Food Program. There is an audio of a word and students have to choose the correct synonym (there's a choice of four). For every correct answer Free Rice donates 20 grains of rice.
The site also rates the level of your vocabulary and sneds you more difficult questions if you are getting the answers.
Great vocabulary building and a worthy charity all in one.
I found it pretty addictive. Try it at Free Rice