The answer to this question depends on what type you want. The following are some commonly used: 1. Google Academic is a search engine, which is used to search academic publications, including papers, articles, books and so on. 2. Baidu Scholar is an academic version of Baidu search engine, providing academic search services in both Chinese and English. 3. The knowledge service platform of China HowNet Knowledge Network Publishing House provides comprehensive Chinese academic literature retrieval and services. 4.VIP is a digital library covering periodicals, conferences, newspapers, papers and other academic resources. 5.Doc88 covers all kinds of subject knowledge, including literature, courseware, papers, technology and other learning platforms. 6. The Library of Congress should be one of the largest libraries in the United States, providing a large number of historical documents, maps, music and so on. 7. Internet archives are online digital libraries maintained by non-profit organizations, which provide historical web pages, movies, music and other types of digital documents. In short, provide different resources and choose the best one that suits your research direction and needs.