Metasploit is described as 'Community Edition simplifies network discovery and vulnerability verification for specific exploits, increasing the effectiveness of vulnerability scanners such as Nexpose - for free' and is an app in the Security & Privacy category. There are more than 10 alternatives to Metasploit for a variety of platforms, including Linux, Windows, Mac, Online / Web-based and Self-Hosted solutions. The best alternative is Nessus. It's not free, so if you're looking for a free alternative, you could try Censys or Armitage. Other great apps like Metasploit are Exploit Pack (Free, Open Source), Social-Engineer Toolkit (Free, Open Source), ZoomEye (Freemium) and Immunity CANVAS (Paid).
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.
Nessus vs Metasploit opinions
Some users think Nessus is a great alternative to Metasploit, some don't.
Nessus scans for vulnerabilities, Metasploit exploits upon the vulnerabilities. I guess the prices are similar though...
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.