These invisible sites are available through several directories already. A directory, of course, is just a listing of sites. You still have to pick the right ones and do the search yourself, using whatever search facility the database offers. It's a good way to waste an afternoon, without having much to show for it. It's in beta test now.
Backers seem to label it as the Internet's largest search portal.
The interface is deceptively simple. Type in a search request and the service will automatically search across multiple sources and categorise the results. It may offer suggestions for additional searches based on what it's learned. You can also search by category, or by source within a category. You can define searches that run every day, and ask it to alert you to changes in pages you specify.