All standard features (on 3GS anyway) so maybe useful to new users of the iPhone for people who may have accessibility needs.

Invert Background Colour to White
(1) Setting
(2) General
(3) Accessibility
(4) White on Black (ON/OFF)
- looks quite cool

Zoom
(1) Settings
(2) General
(3) Accessibility
(4) Zoom (ON/OFF)
- you have to use three fingers on the screen to Zoom

Voice Over
(1) Settings
(2) General
(3) Accessibility
(4) Voice Over (ON/OFF)
- will put a voice over each touch command made

This one can be useful to everyone.
Show Battery Life as %
(1) Setting
(2) General
(3) Usage
(4) Battery Percentage (ON/OFF)
- see the % battery life now on the Apps screen (home screen)