An add-on to Anki that auto-defines words, optionally adding pronunciation and images. Hit up ankisrs.net if you're unfamiliar with Anki and spaced repetition systems.
Note: I use Merriam-Webster's Collegiate® Dictionary with Audio API to get definitions and pronunciations. This requires you sign up for a Merriam-Webster account and use your own individual API access key. Go to the Merriam-Webster Dictionary API website, sign up for an account, and request access to the Collegiate Dictionary. Then, replace YOUR_KEY_HERE with the key you receive.
Licensed under GPLv2.