Subject directories are human created, selective, and arranged by topic. They are good for broad topics and for finding a few good sites on a particular topic.
These are machine generated, non-selective, and search as many web pages as possible. They're good for unique or specific keywords or topics.
Blog & Social Media Search Engines
Meta-Search Engines - cross-search multiple subject directories and search engines.
Beta, Experimental, New, Specialized - a variety of tools and resources
More on web searching & web search tools.