Want to practice your spelling online? Try the Visual Thesaurus Spelling Bee. The first word I got when playing was "host." But by the third word, I was asked to spell "unpropitiously."
After skating through "manumit" and "cinnabar," I got stumped on "reseau" (a grid of fine lines in lacemaking or celestial measurement), then nailed glaucous, jeroboam, and cislunar before striking out on "ecdysis" ( a shedding of the cuticle in arthropods) and "naumachia" (a naval spectacle or mock sea battle put on by the ancient Romans). So, um, it gets hard!
Here's a blog post explaining how Visual Thesaurus "uses the latest computational tools to shed new light on the inner workings of the English language." Complete with graphs!