Artha is a free, open source dictionary and thesaurus. It is surprisingly powerful; its thesaurus is excellent (and arguably better than thesaurus.com). I use it on a regular basis. It is excellent.
The thing that really makes Artha shine (other than the fact that it's entirely offline) is the fact that you can have the window closed, and bind the "define" feature to a key (such as ctrl + something). You can then select any word in any application, webpage, software, etc. and press the hotkey, and artha will display the definition in the corner, in a transparent little notification-esque window that will disappear in a few seconds. It's amazing. It's wildly more convenient than having to open a tab in your browser and searching for the word. Don't get me wrong; googling things is still pretty convenient, but this is simply better.
Its features include:
1) All the definitions for a word, listed in a concise, easy-to-read format. By "concise", I mean "not given a copious amount of spacing between the lines to be more 'aesthetically pleasing'". I personally really like the interface; it's spaced well and easy to read.
2) All synonyms for a word. You can click on a synonym to get all the synonyms for that word, and so forth.
3) What the word is a kind of. For example, the term "dog" returns "canine", "canid", "domestic animal", "unpleasant woman", "disagreeable woman", "chap", "fellow", and a number of other terms. You can click on any of the "kinds" to get their definitions.
4) Derivative words. Pretty self-explanatory.
5) Domains of words. This will list various different topics related to your search; you can click on the topics to see the page (definition, synonyms, kinds, domains, topics, etc.) for that topic word.
There are other features as well, but these are the major ones.
Artha is also available on Linux. I don't know about Mac computers, but I would imagine that if it's available on Linux it's available on Mac as well. I used Linux for many years, so I really appreciate Linux compatibility. I can't remember where I downloaded it from, but upon a great deal of googling, the only download that I can find is here, on SourceForge. Evidently it doesn't have its own website, which I suppose is understandable considering that it is free and open source.