The MiningCo is a good catalog but suffers from three flaws. First, it is very, very, very subjective Second, the quality of an individual area depends on the skills, knowledge, commitment and time available of the Guide reponsible. Great ones make great areas, others don't so the quality is uneven. Finally, this catalog actually keeps you on their site when they link you out, making bookmarking a chore. When you need to search broadly for information sources, but not necessarily documents, you'll want one of the top search engines.
Search engines are software.
Then they take the information from the page and use it for indexing. The process is automatic. Search engines all do their searching, indexing and ranking a little differently, so it pays to try searches with more than one. HotBot is the search engine with the largest index. It's also my personal favorite among the search engines.