Had an opportunity to work with Ontolica with my last client. They have, IMHO, the best third party search tool available. There are a series of not so easy tasks to implement it (mostly, dealing with document tagging, editing schema.xml for each library, etc...), but once up and running, the search results page offer a "drill-down" Web part to allow navigation through the search results. It uses the SharePoint API so it doesn't break anything. The search feature is standard OOTB searching but the product forces the user to apply metadata to every document, hence the vastly improved search results but anyone can do that on their own. Where the product shines is in its ability to manipulate the search results with grouping by area, topic, site, and allowing the user to drill into the respective grouping.