Probably one of the most requested features for DN that we've heard since launch is the addition of a search feature. As a part of the recent DN 2.0 updates, we've done just that.
Take a gander to the link in the sidebar and give search a try. It searches through both post content and URLs. It does not search users or comments at this time.
For the technically inclined, our search is backed by the wonderful elasticsearch (http://elasticsearch.org).