AbuseIPDB - Check for known malicious IP
VirusTotal - Hash lookups for files, URL/Domain/IP analysis, has API-support
ANY.RUN - Cloud-based sandboxing platform for malware analysis, detailed reports on malware behavior—process creation, network communication, system changes
Communities
SOC Prime Threat Detection Marketplace - professionals share threat detection rules, including latest CVEs
MalwareBazaar - For sharing malware samples, set alerts to track newly observed malware
Malshare - For sharing malware samples