Is there a site in the web ( that you might know of ) where it is possible to search for other sites by keywords?

I need to find sites that have various functions, which don’t just appear in everyday surfing.
It hapens every once in a while that I get an idea on sites, i.e. once I was looking for a site where questions and answers flowed.. in the end I found fluther, by pure chance…

Is there a website which acts as a sort of search engine for other sites?
Please don’t give obvious and not fully satisfying things such as ‘google’

EXCLUDING: stumbleupon.