Shodan is the world's first computer search engine that lets you search the Internet for computers. Find devices based on city, country, latitude/longitude, hostname, operating system and IP.
Check out Shodan Exploits if you want to search for known vulnerabilities and exploits. It lets you search across Exploit DB, Metasploit, CVE, OSVDB and Packetstorm with one simple interface.
Check out the official Shodan API documentation and learn how to access Shodan from Python, Ruby or Perl. For an example of how Shodan can offer new functionality to your program, check out the awesome FOCA tool.
The Shodan Research website includes projects that provide new insights and interesting information using the Shodan data or API.
Sign up for a Shodan account or securely login using your Google, Twitter or Facebook account and then try out some popular search queries by browsing the search directory. Oh, and follow me on Twitter if you want to stay up-to-date with Shodan and other cool things I work on.