Slayer's staff

The Slayer's staff is sold by Slayer Masters, it can also be obtained by pickpocketing HAM members after completing the Buyers and Cellars quest. It requires level 50 Magic and level 55 Slayer to wield and is required for the Magic Dart spell. After recent updates, it's not that popular anymore because anyone can Autocast Crumble Undead without the need of this staff or any other.

Many people still use it occasionally, since it's still one of the requirements to cast Magic dart. This makes it useful for doing the Barrows activity and killing Dagannoth Kings. Finally, it is the most powerful (in damage/minute) weapon for level 1 in Attack, so it is a popular choice for obsidian maul pures in the making (if they have 55 Slayer and 50 Magic).

Its popularity has declined now that the Staff of Light has been released, as the Staff of Light has a 15% magic damage increase, can be used to cast Magic Dart, and has a special attack, and even has a chance of not using any runes for Combat Spells. Of course, the fairly high price of the Staff of Light (10 million was just the starting price) makes the Slayer's staff still an attractive option for poorer players, or just those who don't want to expend millions, or those who don't have 75 Magic and Attack yet.

Remember that now you can autocast spells via Spellbook, and you don't need a staff anymore (except the ones required for certain spells).

Trivia

 * A slayer staff can be obtained by pickpocketing a H.A.M. member after the compleion of the "Buyers and Cellars" and the further completion of the two mini-quests "From Tiny Acrons" and "Lost Her Marbles". This is however a fairly rare.