The
XDXF Dictionaries Download page only lists three large freely available English dictionaries, one is Wordnet and the other two are available in the MOBI upload section:
GNU Collaborative International Dictionary of English, V1, 9, July 2007 and
Dictionary, Webster's: Webster's Dictionary 1913, v1, 22 Nov 07. Note that StarDict dictionaries (e.g. from XDXF) can be converted to MOBI, see
How to create your own mobipocket dictionary for any language: (which should perhaps be moved to the Mobi/PRC Books Upload section).