AlienVault OTX (Open Threat Exchange) is a free, community-driven threat intelligence sharing platform — organizations contribute and consume IoCs, malware samples, and threat data via "pulses."
OTX integrates with most SIEMs and security tools. Free tier provides high-value threat intelligence. Contributing back strengthens the community. AT&T Cybersecurity (MSSP) uses OTX as the backend for their SIEM platform.