Cobalt Strike is described as 'is threat emulation software. Red teams and penetration testers use Cobalt Strike to demonstrate the risk of a breach and evaluate mature security programs'. There are more than 10 alternatives to Cobalt Strike for a variety of platforms, including Linux, Windows, Mac, Online / Web-based and Self-Hosted solutions. The best alternative is Metasploit. It's not free, so if you're looking for a free alternative, you could try Censys or Armitage. Other great apps like Cobalt Strike are Nessus (Paid), Social-Engineer Toolkit (Free, Open Source), ZoomEye (Freemium) and Exploit Pack (Free, Open Source).
The Nessus vulnerability scanner is the world-leader in active scanners, featuring high-speed discovery, configuration auditing, asset profiling, sensitive data discovery and vulnerability analysis of your security posture.
The Social-Engineer Toolkit is an open source penetration testing framework designed for social engineering. SET has a number of custom attack vectors that allow you to make a believable attack quickly.
Immunity's CANVAS makes available hundreds of exploits, an automated exploitation system, and a comprehensive, reliable exploit development framework to penetration testers and security professionals worldwide.