Start Your Own Search Engine
Start your own search engine or directory with these tools and services. If your interest is in starting your own pay per click search engine, a crawler based search engine, or a simple directory, you will find the information you need here.
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Oh dear, there's damn too many of those tiny things like that out there in the Internet already and the glory days of useless directories are long over.

I think Google co-op custom search engines offer another good way of offering an industry specific search portal or feature on your website. The directory market is quite saturated but there is still room for industry specific search engines or portals.

That is awfully hard to do.
Even Microsoft is having its troubles against Google.