Excalibur

Excalibur is a very easy weapon to get (if you are lawful). Simply dip a long sword into a fountain, provided you are a lawful character and at least level 5. It's intelligent, so it can only be safely wielded by lawful characters. As a weapon, it does +d5 to hit and +d10 damage. What's equally useful is that it confers level-drain resistance and automatic searching, and makes searching more likely to be successful. On the other hand, it also makes demon princes generated hostile, and every monster can find you, whether it has eyes or not, O Paragon of Virtue.

Strategy

Dipping for Excalibur is easy for Knights and Valkyries who both start with a long sword (although some Valkyries are neutral). Acquiring it is generally a good idea, although the chance of getting it is only 1/6 at each dip. The other 5/6 of the time, normal dipping effects occur, so be prepared to deal with water demons, nymphs and other fountain hazards.

One possible downside to getting Excalibur early is that it increases the number of artifacts in the game, which decreases your chance of successfully wishing for a better one later.

Mythos

Excalibur is the legendary sword of King Arthur, who is also present in NetHack as the Knight's quest leader. See Wikipedia for an in-depth description.

Encyclopedia description

                At first only its tip was visible, but then it rose, straight,
                proud, all that was noble and great and wondrous.  The tip of
                the blade pointed toward the moon, as if it would cleave it
                in two.  The blade itself gleamed like a beacon in the night.
                There was no light source for the sword to be reflecting
                from, for the moon had darted behind a cloud in fear.  The
                sword was glowing from the intensity of its strength and
                power and knowledge that it was justice incarnate, and that
                after a slumber of uncounted years its time had again come.
                After the blade broke the surface, the hilt was visible, and
                holding the sword was a single strong, yet feminine hand,
                wearing several rings that bore jewels sparkling with the
                blue-green color of the ocean.
                        [ Knight Life, by Peter David ]