Related keywords & spelling suggestion API

Google Toolbar Auto Complete

Replace your own "q=keywords": http://google.com/complete/search?output=toolbar&q=Ad+Server

Google SOAP Search API

http://scholar.google.com/apis/reference.html#1_3

Yahoo Related Keywords API

http://developer.yahoo.com/search/web/V1/relatedSuggestion.html

Yahoo Spelling Suggestion API

http://developer.yahoo.com/search/web/V1/spellingSuggestion.html