An athame is probably the most common constructed witch tool.
Like any other tool, it only does you good if you find it works for you.
Athames are traditionally used to direct and store magickal energy. They are almost always personal tools, meaning they are only to be used (or even picked up) by the owner.
Athames traditionally have black handles because black absorbs energy and holds it - just feel any black item that's been out in the sun for a while. Athames are also traditionally double-sided, to remind the user that power cuts both ways (i.e. consider how your magick will affect others, there will be both positive and negative that will come of your actions).