Ref Seek (refseek.com) is a search engine whose intention is to only serve links that have potential academic use. Ref Seek seems to eliminate the advertising and paid links found on Google, Ask, Yahoo, and other commercial search tools.
Yolink (yolink.com) quietly powers the search boxes found on many websites today. Yolink’s technology is included in Sweet Search (sweetsearch.com), a popular tool among teachers.
Google Scholar (scholar.google.com) which indexes published research.