The English thesaurus included with most versions of OpenOffice.org:

en,AU,th_en_US,English (Australia),thes_en_US.zip
en,CA,th_en_US,English (Canada),thes_en_US.zip
en,NZ,th_en_US,English (New Zealand),thes_en_US.zip
en,GB,th_en_US,English (United Kingdom),thes_en_US.zip
en,US,th_en_US,English (United States),thes_en_US.zip


Can be registered in the following manners in dictionary.lst,
depending on the language you usually use:

THES en AU thes_en_US
THES en CA thes_en_US
THES en NZ thes_en_US
THES en GB thes_en_US
THES en US th_en_US

There may be differences but for these, new additions should go
in a smaller unique dictionary.