It is bound to the P key. Finally, it will allow you set the glue and transparency for the paint. Please see the layers section of the page for more information about glue and transparency. Shift -dragging constrains brushing to vertical and horizontal only. Command temporarily activates the path arrow.
Option temporarily activates the eyedropper.
The bottom 8 tools on the tool palette are brush tools, meaning that their effect size can be changed by the brush palette. The brush palette allows for the creation of new brushes. The resultant dialog box is fairly self explanatory, but is limited to making brushes elliptical in nature. Photoshop brushes obviously lack the natural media power of Fractal Design's Painter, but they are flexible enough to suit most retouching needs. For more flexible brushes, any selection can be turned into a brush by using the Define Brush command under the brush palette menu.