Yahoo has through its My Web 2.0, launched a social search engine with TAGs. It’s a beta but looks very good (although I think Technorati Beta does it better).  On the Ysearch Blog , Yahoo explain what social search is:

 It is a new kind of search engine – a social search engine – that complements web search by enabling users to search the knowledge and expertise of their friends and community in addition to the web.

And how it is different:

Social search complements web search, which is driven by publishers and web sites, by providing a better search experience that is powered by people and communities. Flickr is a great example of this power applied to photos and image search.

and their vision for the service:

Over time, we envision communities using My Web to build their own search engines to capture and make accessible the knowledge of their community – search engines populated with the collective experience of a group of medical researchers, a community of PHP experts, a bird watching club, or members of a structural engineering consulting firm.

Looks really great and Im sure many others will follow…