What Search Engines Do

• Web Crawling
• Indexing
• Searching

Popular Search Engines

• Google
• Yahoo
• Ask
• Live Search
• Altavista (based on Yahoo)
• Alltheweb (based on Yahoo)
  

Specialized Search Engines By Topic or Type of Information

• Monster.com (jobs)
• Pubmed (medical)
• Topix (news)
• Ex.plode.us (people)
• Zillow (real estate)
• Bookfinder (books)
• Technorati (blogs)
• LISZen (library blogs)
• Findsounds (sounds)
• Findarticles (magazine articles)
  
• Flickr (photos)
• Truveo (videos)
• Mapquest (maps)
• Shopping.com (products)
• Whitepages.com (phone #s) 
• Yahoo Answers (Q&A)

Meta Search Engines

• Clusty
• Dogpile
• Metacrawler
• Carrot Clustering Engine
• Intelways (sort of a metasearch engine)

Visual Search Engines

• Kartoo
• Grokker
• Quintura

Web Directories

• Yahoo Directory
• Open Directory Project
• Librarian's Internet Index
• Mahalo
• Refdesk
• Hotsheet

More Information

• Search Engine Watch
• Search Engine Guide
• Search Engine Showdown
  
• Yahoo Directory of Search Engines and Directories
• Top 100 Alternative Search Engines (from ReadWriteWeb)
  
• Web Search Engines (Wikipedia)
  
• Web Directories (Wikipedia)