Ninjawords is described as 'Ninjas are three things: Theyre smart, theyre quick, theyre deadly accurate—Ninjawords is a dictionary for the iPhone built on these principles' and is an app in the Education & Reference category. There are more than 25 alternatives to Ninjawords for a variety of platforms, including Online / Web-based, Android, iPhone, Windows and Linux. The best alternative is GoldenDict, which is both free and Open Source. Other great apps like Ninjawords are Wiktionary (Free, Open Source), WordReference (Freemium), Dictionary.com (Free) and Merriam-Webster (Free).
GoldenDict is an open-source dictionary program for PC computers that gives translations of words and phrases for different languages. It allows use of several popular dictionary file formats simultaneously.
Google's dictionary extension on Google Chrome. With it, you can double-click any word to view its definition in a small pop-up bubble, or search the complete definition of any word or phrase using the address bar dictionary.
Online Dictionary - Multiple dictionaries including: English dictionary, medical dictionary, legal dictionary, financial dictionary, computer dictionary, thesaurus, dictionary of acronyms and abbreviations, dictionary of idioms, thesaurus, Columbia encyclopedia, Wikipedia...
Reference.com offers a massive online dictionary, thesaurus, encyclopedia, quote finder, and translator (powered by Google Translate)- all for free. For the Dictionary.com and Thesaurus.com mobile app, go here. Dictionary.com
Artha is a handy thesaurus based on WordNet with distinct features like global hotkey look-up, passive desktop notifications, regular expression based search, etc.. Artha may be used as a free open-source replacement to the proprietary WordWeb Pro.