Masscan

Masscan is an Internet-scale port scanner, able to scan the entire Internet in just 6 minutes or less. It's considered the fastest port scanner and is similar to the famous port scanner nmap, and like port scanners scanrand and unicornscan it uses asynchronous transmissions. However, it is more flexible when it comes to arbitrary port and address ranges.

Masscan not only detects when ports are open, it can also check banners. And while it is useful for smaller, internal networks, it's designed with the entire internet in mind. Do take note however that Masscan uses its own custom TCP/IP stack, so anything other than a simple port scan could cause conflict with the local TCP/IP stack unless certain configurations are done.