Note that this will only work properly for categories whose index pages are visible to the public (ie to anyone not logged in) and for searches that return publicly visible items.
To access non-public items an RSS client would need to be able to supply user-definable cookies for authentication, and I'm not aware of any such at this time.
If you want to access the headlines from the front page of the site via RSS, use:
If you want to take a feed of all the most recently posted public items throughout the site, use:
Different versions of RSS are available, the default is RDF/RSS 1.0 but you can specify which one by passing the version number as the rss parameter eg:
...&rss=1.0 (same as rss=1)