Excuse the sloppy copy paste from something I was working on. Note I've not checked the urls in some time and the descriptions were mostly automated.

Literary Search Engines

• ⭐️BASE: One of the world's most voluminous search engines especially for academic web resources. You can access the full texts of about 60% of the indexed documents for free (Open Access). BASE is operated by Bielefeld University Library.
• CAS SCIFINDER: More than a literature and chemical compound database, CAS SciFindern leverages human curation to deliver relevant, actionable insights so you can innovate faster.
• GET THE RESEARCH: Clean, simple academic search engine with an open access toggle. Leverages Open Access and AI to help nonspecialists find, read, and understand scholarly research.
• FATCAT!: Fatcat is a versioned, publicly editable catalogue of research publications: journal articles, conference proceedings, pre-prints, blog posts, etc. The goal is to improve the state of preservation and access to these works by providing a manifest of full-text content versions and locations.
• IEEE XPLORE: The IEEE Xplore digital library is a powerful resource for the discovery of scientific and technical content published by the IEEE (Institute of Electrical and Electronics Engineers) and its publishing partners.
• INSPIRE HEP: INSPIRE is a trusted community hub that helps researchers to share and find accurate scholarly information in high energy physics.
• ⭐️INTERNET ARCHIVE SCHOLAR: This fulltext search index includes over 25 million research articles and other scholarly documents preserved in the Internet Archive. The collection spans from digitized copies of eighteenth-century journals through the latest Open Access conference proceedings and pre-prints crawled from the World Wide Web.
• LENS: The Lens serves integrated scholarly and patent knowledge as a public good to inform science and technology-enabled problem-solving.
• NIH ICITE: iCite is a tool to access a dashboard of bibliometrics for papers associated with a portfolio. Users type in a PubMed query or upload the PubMed IDs of articles of interest. iCite has three modules: Influence, Translation, and Open Citations.
• ⭐️OPENALEX: FOSS catalog of the global research system featuring API access for developers.
• PAPERSWITHCODE: The mission of Papers with Code is to create a free and open resource with Machine Learning papers, code, datasets, methods and evaluation tables.
• The SAO/NASA Astrophysics Data System: The ADS maintains three bibliographic collections containing more than 15 million records covering publications in astronomy and astrophysics, physics, and general science, including all arXiv e-prints.
• SCINAPSE: Our tool extracts valuable information such as related topics, companies, and patent data from each paper. Through our database, we provide a more comprehensive research experience.
• ⭐️SEMANTIC SCHOLAR: A free, AI-powered research tool for scientific literature.
• ⭐️WEB OF SCIENCE: The Web of Science contains a remarkable treasure of data on scientific content, impact, and collaborations from 1900 to the present day on a global scale.
• WORLDWIDESCIENCE: WorldWideScience.org is a global science gateway—accelerating scientific discovery and progress through a multilateral partnership to enable federated searching of national and international scientific databases and portals.

Literature Mapping

• CITATION GECKO: Gecko helps you find the most relevant papers to your research and give you a more complete sense of the research landscape.
• CODA: Use Coda, a machine-readable history of cooperation research, to search, select and visualize studies for on-demand meta-analysis.
• CONNECTED PAPERS: To explore connections between published papers (e.g., for a literature review), use Connected Papers. This is also nice to see the reach of your own research! It is a "unique, visual tool to help researchers and applied scientists find and explore papers relevant to their field of work".
• CONSENSUS: A search engine that uses AI to extract and summarize findings from scientific work.
• COVIDENCE: This paid tool is well worth its money, because it helps you screen and decide on hundreds of papers if you're working on a systematic review.
• DIMENSIONS: Dimensions is a linked research knowledge system that re-imagines discovery and access to research. Developed by Digital Science in collaboration with over 100 leading research organizations around the world, Dimensions brings together grants, publications, citations, alternative metrics, clinical trials, patents and policy documents to deliver a platform that enables users to find and access the most relevant information faster, analyze the academic and broader outcomes of research, and gather insights to inform future strategy.
• ELICIT: Elicit is your AI research assistant that "uses language models to help you automate research workflows", by finding "relevant papers without perfect keyword match", summarizing "takeaways from the paper specific to your question", and extracting relevant information.
• INCITEFUL: AI tool that analyses a network of academic papers to help you discover the most relevant literature.
• INTRIGUE: Intrigue lets you quickly organize the papers you read alongside your thoughts in a visual & clean manner. Check out the web demo, which also serves as a tutorial.
• LITERATURE-MAP: The Literature-Map is part of Gnod, the Global Network of Discovery. It is based on Gnooks, Gnod's literature recommendation system. The more people like an author and another author, the closer together these two authors will move on the Literature-Map.
• LITMAPS: An all-rounder tool for visual research navigation, citation network search, and team synchronization.
• OPEN KNOWLEDGE MAPS: A tool which creates a topical overview for your search query based on the 100 most relevant documents.
• PUBREMINER: Detailed analyses of PubMed Search results.
• PUBVENN: PubVenn enables you to explore PubMed using venn diagrams. Enter any multi-term search above to see the relative size of the citation set for each term as well as how those sets interact.
• RESEARCH RABBIT: Use Research Rabbit to find both germinal and future works from a single (or multiple) works.
• SEMANTIC SCHOLAR: A free, AI-powered research tool to find scientific literature.
• SNOWBALL: A tool that would find related articles with snowballing method.