Firefox extension offers an addictive way to search via categories or keywords or happenstance; you train it, with thumbs up or down on sites, so it learns your tastes; I prefer searches on keywords so far, and have had little desire to "network"
Taggound is a social search engine mashup. What does that mean? Tagground searches various social web applications (ex. yahoo my web 2.0, del.icio.us, raw sugar, flickr, You Tube etc) repositories to retrieve relational data (ie tags, urls) that other use