The Vocabulary Builder at UnivSource is in beta phase but it is a nice tool to practice and learn for GRE. It displays the meaning of a word in detail and then presents a few choices to select an appropriate antonym from.
Direct Link: http://www.univsource.com/vbbeta.php

I found this great website for vocabulary building where you get to help the poor around the world by building your vocabulary. Amazing concept!!
The website presents you with a word and four choices to select the closest synonym. If you get it right, you automatically donate 20 grains of rice through UN (the sponsors pay for that). And the whole system is adaptive i.e. if you get it right, you get a more difficult word next.
It is, indeed, a great resource for honing your GRE vocab. It is packed with …