Pattern
A number of interesting effects can be achieved using the flood fill tool. Every fill style, except for the solid fill, has option settings.

Linear
The options settings determine the directon and angle of the foreground and background colors in the gradient fill, and allow you to select a pattern for the patern fill.

Rectangular
Setting your gradient fill options gives the effect of light and shadow. Reversing the foreground and background colors along with the option settings determines which direction the light is coming from.

Sunburst
Combined with other tools, this can be utilized to create a number of different and varying images.

Radial
Combining a Radial Gradient Fill with a Pattern Fill, tolorance set to 50 will produce this image . Fill your window with the Radial Gradien Fill, change the options to pattern, tolorance to 50, and without making a selection, click over the area you want the pattern to be in.