After much searching, I have decided that http://dict.org is the best free on-line dictionary.

Here's a BookMarklet for handy access:

 javascript:var termToLookUp = prompt("Look up which word?", ""); if (null != termToLookUp) {location.href = "http://www.dict.org/bin/Dict?Form=Dict2&Strategy=*&Database=*&Query=" + termToLookUp;}