Ask: Offers search for web sites, images, news, blogs, video, maps and directions, local search and shopping

Bing: Search engine developed by Microsoft. Features web, image, video, local, news, and product search

Google: Enables users to search the world's information, including webpages, images, and videos. Offers unique features and search technology

Google AJAX Search API Blog: Official blog. Provides news and tips about this API

Google Web Search API: Official site. Includes developer's kit, terms and conditions, and FAQ

Sitemaps.org: Information on the Sitemaps protocol, a common standard of major search engines that enables webmasters to inform engines about pages on their site that are available for crawling

The Web Robots Pages: Information on the robots.txt Robots Exclusion Standard and other articles about writing well-behaved Web robots

Yahoo! Search: Web search engine also indexing images, video, shopping sites, and local results