English synonym online database?

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.

http://www.synonym.com/ - in their own words:

Synonyms Thesaurus with Antonyms & Definitions

Synonym.com is the web's best resource for English synonyms, antonyms, and definitions. Type in a word and the synonym finder will come up with a list of synonyms. Check out the most popular online synonym searches to find the right word for any occasion. Use the antonym tool to find find antonyms and check out the definitions feature to read the definition.