masscan

Masscan is a TCP port scanner which spews SYN packets asynchronously, scanning entire Internet in under 5 minutes.
It produces results similar to nmap, the most famous port scanner.

Internally, it operates more like scanrand, unicornscan, and ZMap, using asynchronous transmission. The major difference is that it's faster than these other scanners. In addition, it's more flexible, allowing arbitrary address ranges and port ranges.