Nmap has established itself as a premier, free, open-source network mapping tool. There are alternatives that are also free, though they vary in their sophistication and ease of use.

The Nmap Project just released the Holiday Edition of its open source cross-platform security scanner and network mapper, with several important improvements and bug fixes. New features in Nmap 7. ...

Nmap doesn't perform vulnerability assessment; it's only a port scanner. If something more sophisticated is needed to spot flaws, consider using an automated Nessus-based approach instead.