Lots of various search engines. Some you don't know about!

Google: Duh.

Wolfram's Alpha: A new type of search engine. It gives you access to the world's facts and data and calculates answers across a range of topics, including science, math, economics, politics, .... This is part of what is behind Apple's Siri software.

Factual: Collection and collation of lots and lots of "facts", also contains links to data sets on things like education and government statistics.

LexisNexis Library Express API: This service allows users to search for and access over 10,000 full text news, business, and legal publications, including backfiles, recent national and regional news, wire services, broadcast transcripts, international news, and non-English language sources.

The Wayback Machine: Over 150 billion web pages archived from 1996 to a few months ago.  Useful if you are trying to scrape historical data that seems no longer available, for example.

Pages for the Profession

Scholarometer: This service (available as an API or browser extension) provides impact analysis estimates of resources retrieved, using a citation analysis function that totals appearance of one resource in reference lists of others.  It also collects information about authors and disciplines, including their interrelationships.

SubSift: An API to support academic peer review through submission sifting -- connecting conference or journal papers to potential peer reviewers using papers written by the peer reviewer.