The Banes are a group of artifact weapons, notable in that they do double damage to a specific group of monsters. They are generally considered to be worthless for most purposes, since they reduce the probability of receiving a more useful artifact. Despite its name, Magicbane is not included, for a couple of reasons: it has no double damage bonus, and it is actually useful. Likewise, Fire and Frost Brand are not included, as their range of speciality is very large, while Sunsword has several other abilities as well.
Demonbane is a lawful long sword that deals double damage to proper demons, and has +1d5 to hit them. It acts as a normal long sword against non-demons, including the Riders, the mail daemon, sandestins, and djinn. Its utility or lack thereof depends on your opinion of the difficulty of most demons.
Giantslayer is a neutral long sword that has +1d5 to hit and does double damage against proper giants, which include giant mummies, giant zombies, and even Cyclops and Lord Surtur, but not ettins or titans.
Sting is a chaotic elven dagger that provides +1d5 to hit, and does double damage to orcs, including orc mummies and zombies. It also reveals the location of orcs on the level, and cuts through spider-webs.
Sunsword is a lawful long sword that does double damage to all undead creatures: L (liches), M (mummies), W (wraiths, wights and Nazgul), Z (zombies and skeletons), ghosts, and shades. It is made slightly more useful by the fact that it's a light source, with a radius of 2, and that it prevents blinding from light-based sources. Also, its damage range is somewhat wider and more useful than that of the other Banes, especially since many spellcasters are undead. The keep-or-toss decision is based on how much you hate undead.
Orcrist, the big brother to Sting, is a chaotic elven broadsword that gives +1d5 to hit and double damage only against orcs. Sting and Orcrist may be created by #naming a single elven dagger or broadsword.
Werebane is an unaligned silver saber. It does double damage against all werecreatures, as well as +1d5 to hit. However, it is slightly more useful than the other banes in that when wielded, it protects you from lycanthropy.
Some non-artifacts with similar names: