Eyeball

1) Create a circular selection (hold shift while using the elliptical selection tool).
2) With white (#FFFFFF) as your foreground color, and your chosen eye color as your background color (#0000D7 in this example), fill your selection using the radial gradient fill tool.
3) Apply the Noise -> Add Noise filter with these settings: Amount:10, Distribution: Gaussian, Monochromatic
4) Apply the Blur -> Radial Blur filter with these settings: Amount: 50, Blue Method: Zoom, Quality: Best
5) Apply the Render -> Lighting Effects filter with the default settings.
6) Draw a small black circle in the center of your eye. This will be the pupil, obviously.
7) Draw a small, white, circular highlight above and to the right of the pupil.
8) Play with the brightness / contrast until you have what you want.