The American Heritage Dictionary, 3rd Edition, On-Line. The standard dictionary.
Different relations link the synonym sets.
Very Cool Specialized Dictionaries Dictionary of Cell Biology Contains the text of the Second edition of the Dictionary of Cell Biology , published in April 1995, together with enhancements, hypertext links and new entries which are destined for the third edition. This site provides useful links to medical, science and life science dictionaries. Computing Dictionary A searchable dictionary of acronyms, jargon, programming languages, tools, architecture, operating systems, networking, theory, conventions, standards, mathematics, telecommunications, electronics, institutions, companies, projects, products, history, in fact anything to do with computing. A good bare-bones searchable database of philosophies and philosophical terms. More than just a glossary or a list of figures of speech, this site is being carefully cross-referenced and edited to show relationships and groupings of terms within and across the various levels of the rhetorical curriculum.