From Metasploit Unleashed - Mastering The Framework
The Social-Engineer Toolkit (SET) is specifically designed to perform advanced attacks against the human element. Originally this tool was designed to be released with the http://www.social-engineer.org launch and has quickly became a standard tool in a penetration testers arsenal. SET was written by David Kennedy (ReL1K) and with a lot of help from the community in incorporating attacks never before seen in an exploitation toolset. The attacks built into the toolkit are designed to be targeted an focused attacks against a person or organization used during a penetration test.