One of those standard tools for any real network or security tech. Also one of those dual edged swords as in the wrong hands it is a well known hack tool. That said, it is well known, trustworthy, and safe to use, just use it legally and ethically as it was meant to be.
The home of one of the staple softwares used for security and network technical people. Also, the opposite applies, it can be used to run scans for not so friendly purposes. Good read and lots of nice information if you're interested in security.