So I was poking around the Internet (as I am wont to do when I'm procrastinating) and I found the Free Dictionary's Free Website Content page and there is all sorts of awesome English related stuff there to embed in web pages, including a dictionary look-up, "idiom of the day", and word games.
I use The Free Dictionary as a reference for ELL often, because it consolidates definitions from other dictionaries and I think stating something several different ways is helpful when trying to explain something. I would love to see the Dictionary Look Up Box available in the sidebar (or maybe just on the "Ask Question" page). If you click on the "Customize" button and select "Simple", it can be quite compact.
I have a feeling that including off-site dynamic content like that isn't feasible, but you can't be sure unless you ask, right?
As for whether the Free Dictionary is on the up and up, their terms of service has some information:
Third-Party Information: Some articles, including the content from Wikipedia®, are distributed under the terms of GNU Free Documentation License. We always strive to make TheFreeDictionary.com the best dictionary resource on the World Wide Web. We sometimes provide users with access to multiple sources of data when accessing our services and provide attribution to such sources in accordance with the relevant law and arrangements with third parties. Some information on our site is provided by third parties and edited by our staff. Check the copyright notice immediately following such articles for more information.
Considering that each entry has a citation already written out in APA, Chicago, and MLA style, and they've been around for 13 years without anyone suing them, I don't think Farlex is any shadier than your typical ad-supported website.