Antonyms are words with opposed meanings. They are the contrary of synonyms, since synonyms are words that share the same meaning.
So the word "antonym" is itself an antonym of the word "synonym". And the word "synonym" is an antonym of the word "antonym", because antonymy is a symmetrical relation: when A is the opposite of B, then B is the opposite of A.
Of course, the meaning of most words cannot be negated, so most words in the language do not have any antonyms.
As a by-product of HyperDic, we have produced a standard text file containing all the antonyms from the latest full version of the dictionary. Through the following link, you can freely download the Antonym Dictionary.
The file is compressed with zip, and fills only 80 Kb. Uncompressed, it fills 400 Kb, and can be read with any standard text reader.
The antonyms found in HyperDic were derived from a slightly modified version of WordNet, where antonymy was expanded from a relation between words, to a relation between synonym sets. This is a sound logical inference, which yields many new antonym pairs, because the synonyms of a words' antonyms are also antonyms of that word.