Maybe some of the other customers would have good insights to this, but I do know that some folks in the past have used our Search API to plug into what search tool that they have used. That at least makes it comprehensively cover the community end of things. If you need more info on that, contact your Lithium CSM.
Sorry I could not be more of a help on recommending an actual product out there in the marketplace that will suit all your needs.
You might also want to check Scott's breakdown of Search here: 3 kinds of search
The Search API JakeR mentioned is good for getting results that incorporate message metadata (tags, accepted solutions, kudos) as cirteria for relevancy of the results. But another benefit to an API approach across content (as opposed to indexing) is the ability to pass authentication information to include results appropriate to the access level of the requester (which sounds like something you are interested in).
Here's a public example of one of our customers using integrated search with their site: http://forums.logitech.com. Once there, search for something on their site and you will see results from both their CRM/KB and forums.
I don't have direct experience with the various search tools out there, but I'm pretty sure that SalesForce.com at least has similar APIs to use for searching content. If you come across any vendors, go ahead and post them here for the community!
You maybe interested in a federated search solution where using APIs we can hook into your existing website search engine to also make community content searchable, or you could use a search appliance to pull in information from multiple sources.
You can easily get a personal demo and more detailed explanantion of the various search options Lithium can help you integrate with by getting in touch with us at the bottom of the contact Lithium page.