Semantic Scholar
The most useful free academic search on the open web, and the only one that also hands you its corpus and its API. Coverage is broad but metadata quality varies by publisher, so it will not replace Scopus or Web of Science for a systematic review. Treat the TLDR summaries as triage, not as a reading substitute.
Pros
- 237M+ papers searchable without an account
- TLDR one-line summaries generated per paper
- Semantic Reader shows citation context inline
- Free Academic Graph API and the open S2ORC corpus
Cons
- Metadata and full-text availability vary by publisher
- No systematic-review tooling such as PRISMA exports
- Non-profit funding means feature pace is not commercial