"Hello Sailor" is the catchphrase easily recognized by legions of Zork fans. This phrase, originally used by hookers to grab the attention of -- you guessed it, sailors, made it into the entire trilogy of Zork works. Generally, the phrase is useless, provoking a "Nothing happens here." It became something of a running joke among Zork fans for a long time, and is easily the most famous phrase of the series. (Although "Hello Footpad" occasionally gets a clever response too.)
In Zork 3, you must wait for a Viking ship. If you say the phrase before it arrives, you are told, "Nothing happens yet." -- after, "Nothing happens anymore." But if you should happen to say it to the Viking ship...
>HELLO SAILOR
The seaman looks up and maneuvers the boat toward shore. He cries out "I have waited three ages for someone to say those words and save me from sailing this endless ocean. Please accept this gift. You may find it useful!" He throws something which falls near you in the sand, then sails off toward the west, singing a lively, but somewhat uncouth, sailor song.
Incidentally, there is also a sailor at the beginning of "Beyond Zork", to whom you can also gain a useful item by saying this phrase.