Learn English – the synonym of “spy”

Google has many results for such a query, though quality sites may be buried under other sites that are better marketed (or more engaged in search engine optimisation).

One main remark: what you are looking for (listings of synonyms and related words) is called a thesaurus (plural: thesauri or thesauruses).

Now, for the sites themselves, I myself am quite fond of:

• The Merriam-Webster thesaurus
• The French CNRS online synonym database, which is part of a larger semantic atlas worth exploring:
• If you own a Mac computer, the Dictionary application (available for free with the operating system) includes the Oxford American Writer’s Thesaurus, 2nd edition. I use it extensively.

paranym, which is defined as

a word or words whose meaning is altered to conceal; an evasion; see euphemism (by Wordnik)

and

euphemism; word whose meaning altered to conceal evasion (by Phrontistery)