General

Need to get started with a broader search? These academic search engines are great resources.

RefSeek: With more than 1 billion documents, web pages, books, journals, newspapers, and more, RefSeek offers authoritative resources in just about any subject, without the mess of sponsored links and commercial results.
Digital Library of the Commons Repository: Find international literature, including free and open-access full-text articles, papers, and dissertations.
OAIster: Search the OAIster database to find millions of digital resources from thousands of contributors, especially open-access resources.
Internet Public Library: Find resources by subject through the Internet Public Library’s database.
Wolfram|Alpha: Using expert-level knowledge, this search engine answers questions, performs analysis, and generates reports.

Want the best of everything? Use these meta search engines that return results from multiple sites all at once.

Dogpile: Combines results from Google, Yahoo!, and Bing, with categories like Web, Images, Video, and White Pages.
MetaCrawler: Allows you to “search the search engines,” returning results from Google, Yahoo!, and Bing.
Mamma: Searches multiple engines and even includes Twitter and job postings.

Resources like the Library of Congress offer incredible archives and documents online. Use these tools for access:

Library of Congress: Searchable source documents, historical photos, and amazing digital collections.
Archives Hub: Access archives from nearly 200 institutions in England, Scotland, and Wales.
National Archives: Historic documents, research, government information, and more from the National Archives.
arXiv e-Print Archive: Cornell University’s arXiv.org offers open access to e-prints in math, science, and related subjects.
Smithsonian Institution Research Information System: Search over 7.4 million records from the Smithsonian’s museums, archives, and libraries.
CIA World Factbook: Offers major reference information on history, government, economy, and more.
State Legislative Websites Directory: Search legislatures from all U.S. states, DC, and Territories for bills, statutes, and legislators.
Catalog of U.S. Government Publications: Search descriptive records for historical and current U.S. government publications.

Resources like the Wikicfp, callforpaper.org offer incredible archive of Call For Papers in various categories.

Wikicfp: WikiCFP is a semantic wiki for Calls For Papers in science and technology fields. There are more than 100,000 CFPs on WikiCFP. Over 100,000 researchers use WikiCFP each month.
CallForPaper.org: Browse upcoming academic events by deadline, topic, or location.

Use these tools to find books, journals, and articles online or in libraries worldwide.

WorldCat: Access items from 10,000 libraries worldwide, including books, DVDs, CDs, and articles.
Google Books: Search millions of books, including free titles and previews.
Vadlo: A life sciences search engine with protocols, tools, and forums for scientific discovery.
Open Library: Offers classic literature, open e-books, and curated lists of resources.
Online Journals Search Engine: Discover scientific journals, articles, research reports, and books.
Google Scholar: Specializes in scholarly resources, including articles, patents, and legal documents.
Bioline International: Managed by scientists and librarians, this database connects you to a variety of scientific journals.
Directory of Open Access Journals: Provides access to quality-controlled scientific and scholarly journals for free.

Important Search Engines Every Researcher Should Know