Scilit
A fast, free scholarly index — MDPI's comprehensive metadata search.
Tier 3 Open AccessFree
Research Database Directory
Every research database, reviewed.
Semantic Scholar
The AI-native index that reads papers so you don't have to read all of them.
Tier 2 Open AccessFree
SocArXiv
The open social-sciences preprint server.
Tier 3 Open AccessFree
zbMATH Open
The free, open index of the mathematical literature.
Tier 2 Open AccessFree
Zenodo
CERN's catch-all repository for any research output.
Tier 2 Open AccessFree
Research Square
The large multidisciplinary preprint platform.
Tier 3 Open AccessFreemium
Retraction Watch
The database of retractions — know when a paper has been pulled.
Tier 2 Open AccessFree
LILACS
Latin American and Caribbean health literature — the regional index PubMed misses.
Tier 3 Open AccessFree
NASA ADS
The free digital library of astronomy and astrophysics.
Tier 2 Open AccessFree
OpenAIRE
Europe's open-science graph — research linked to the funding behind it.
Tier 2 Open AccessFree