HASSET Provides direct term search followed by hierarchical and other types of browsing. Based on a UNESCO thesaurus.
http://www.data-archive.ac.uk/search/hassetSearch.asp - [ Report Dead Link ]
Historical Thesaurus of English Contains the vocabulary of English from the earliest written records to the present, with known recorded dates of usage, taken from the Oxford English Dictionary.
http://www2.arts.gla.ac.uk/SESLL/EngLang/thesaur/homepage.htm - [ Report Dead Link ]