A9 (beta version of amazon.com search engine)
All the Web (search index merged with Yahoo Mar. 2004)
Alta Vista [advanced search] (search index merged with Yahoo Mar. 2004)
BASE (Bielefeld Academic Search Engine)
Citebase (experimental citation index for the free, online research literature)
Clusty (offers clusters of further results)
Colossus
Eccellio (science miner)
Gigablast
Google [advanced search] / Google Scholar / Google Book Search
HotBot [advanced search]
Internet Address Finder
Kartoo
(results are displayed graphically rather than listed)
Lycos Catalog of the Internet
MetaGer (deutsche Metasuchmaschine)
Mooter (visualizes search results through clustering)
MultiCrawl
Quintura (search results are clustered rather than listed)
RedLightGreen (experimental service to bridge between online documents and the documents which cite it)
RedLightGreen (search engine for books)
Scirus (Elsevier search engine for scientific information)
Scopus (Elsevier-based)
Search.com
Seekport.de (deutschsprachige Suchmaschine)
Soople.com (Expert Google search made easy)
Ujiko (Results are graphically displayed rather than listed)
WebCrawler
Windows Live Academic
Wolfram Alpha
Yahoo

Meta-search engines
Booksearch x 3 (simultaneously searches inside books from A9.com, Google.com and MSN Live Search)
ixquick
Quickbrowse
Search Engine Watch
Web Searching Tips