Show categories in search result

Hi ya

When performing a search on my blog, then I want it to show what categories that contains posts, which matches the search-query.

I want to use this category information, so when clicking the link of a category in the search-result, then it will limit the search to that category alone. And when displaying posts that matches a certain tag, then one can give the user the option to only display tagged posts within a matching category.

I know I can get the category of a post within the loop, but I want to list all the matching categories before entering the search result loop.

Is there an existing simple hack or plugin which can provide this kind of functionality ?

Thank you in advance

-Rolf