Cognition Technologies offers a free semantic search engine pulling results from Medline abstracts. They call it Semantic Medline, or Medline.Cognition; it has two URLs, http://www.semanticmedline.com/ and http://medline.cognition.com/ . The help page delineates the proper use of capitalization, quote marks, Boolean, proximity, pattern matching, wildcards, and required vs. optional search words for Cognition searching.
An interesting feature: a set of dropdown boxes appearing on the results page that allow the user to tweak the search by selecting a more appropriate meaning for a search term, where necessary. For example, the search term “pain” has three meanings: unpleasant physical feelings (the default choice); vexatious person/hassle/annoyance; and unpleasant emotional experience. A “use all” option is also available.
My sample searches using Semantic Medline sometimes retrieved more than a matching PubMed Boolean search, and sometimes less. It appears that foreign-language articles are not included in Semantic Medline.
Thanks to David Rothman for the pointer -