Windows: Big boxes of Rosetta Stone CDs and language dictionaries sit untouched more often than not due to their imposing heft. EasyWords just asks you a multiple choice or write-in question every so often, helping you learn or hone a language.
The languages EasyWords offers by default are limited, at the moment, to English, German and Turkish, by default, but you can add in your own language dictionaries. The app can scale the difficulty of questions, up to the point where you're training to teach English as a foreign language. There's also a fairly sophisticated teaching algorithm, which drops questions answered incorrectly more frequently on users.
EasyWords generally sits in the system tray, pinging you every three minutes (or another variable interval) with multiple choice or written answer questions. You can also click and dig into the word database for quick word look-ups as you study.
EasyWords is a free download for Windows systems only. If you've got a preferred language tool that really helped you to learn at your desk, do tell us about it in the comments.