All the Web - http://www.alltheweb.com/
All the Web, with the current release, has over 300 million searchable documents, and will continue to grow with the Web to 1 billion documents and beyond. It has customizable searching features and advanced searching options.
Alta Vista - http://www.altavista.com/
Alta Vista is a fast search engine with one of the two largest indexes on the web (about 100 million). A favorite of researchers because of its large index and advanced search capabilities, it has also recently added subject-style searching. Excellent instructions.
Britannica Internet Guide - http://www.britannica.com/
Britannica's magazine-style look and emphasis on fun and current topics make it friendly to use. It's a good place to start when looking for popular topics. Britannica also allows keyword search through its topics.
BUBL Link - http://bubl.ac.uk/
This excellent British-based subject directory allows you to either search or browse through its index. It specializes in being the Web's best Dewey Decimal System catalog, but you can also hunt by such things as subject, author or abstract. Focused mainly on higher education topics.
Digital Librarian - http://www.digital-librarian.com/
A great place to start. Digital Librarian is an example of how subject directories began on the web-with people accumulating worthwhile sites and listing them by subject. In this case, the accumulator is Margaret Anderson, a librarian in New York State.
Excite - http://www.excite.com
Excite, with about 60 million in its index, is among the more popular search engines. It has advanced searching options such as complex, keyword, and natural language searching, and allows you to explore topics related to your search.