Here’s an interesting, albeit quirky, search program called, suitably enough, Mercurius. From the blurb:
When you perform a search using Mercurius, as well as the usual search engine results you are shown lists of words and phrases found within these results. So, if you feel that the returned sites are not exactly what you wanted, you can iterate the search using any of the listed words and phrases that you think are relevant, and continue this iteration until you have found exactly what you want.
Basically Mercurius will search for you, then list all the words and phrases inside those results, allowing you to form a more specific search. It should be great for those times when you’re trying to find something where the keywords are not specific enough. Mercurius is available from Silvawood Software, an apparently one-person operation in the UK.