Hi
LazyNezumi is the way to go if you are on Windows, as Ged says, but I am not (I wish it ran on Mac!)
If you want to use a constrain a brush stroke to a straight line or perfect circle without using the Pen tool in PS, follow these steps:
- Create or select the layer where you want to apply the paint.
- Use the Ellipse tool for a circle or the Line tool for a line. In the options bar, set the mode to Path (important!)

- Hold the Shift key as you draw.
Press the spacebar to move the shape as you are drawing (pans image if mouse is released).
Use the Path Selection tool to move the path.
Use the Direct Selection tool to reshape the path.
- Set the Brush to the shape, size, and color that you want to apply to the path.
- In the Paths panel menu, choose Stroke path and select Brush > OK to apply the brush stroke to your path in the shape of the circle (or straight line).

Note that you can save your paths and you can have more than one in your file. Once you apply the brush to the path, they are not connected. You have the brush stroke on the layer and the path can be moved, scaled, and used again without changing the pixels.
~ Jane