Keotag is a tag search engine, but it only searches one engine at a time. You enter a query and then see the results that that were tagged for the term on Technorati, Furl, Yahoo, and more than a dozen other sites, but not all at once.
The trick with meta-tag searching is to integrate and rank the results rather than segregate them by source. Even a tabbed approach by content type would be OK. As in returning tagged results for:
web | video | blogs | images, etc.
Providing a UI like Keotag is kind of helpful, but it’s not as helpful as I’d like. To build a real meta-tag search engine requires some tricky indexing and a nifty ranking algorithm. As well as related search prompts, stemming, wildcarding, etc. You know, a meta-search engine.