HubMed has a new query expansion feature this week: clicking the '+' next to the search terms will retrieve a set of terms—those that are most highly represented in the first 500 documents matching the search terms, relative to the frequency of those terms in all documents—which can then be clicked on to add them to the query. A option to use boolean AND or OR, to narrow or widen searches respectively, is also available.
Here's a screenshot:
