Problem:

Not all input devices have a mouse wheel for zooming or a functional equivalent. 

Rationale: 

Simple buttons (+/-) to zoom in and out in the UI are easily discoverable and are non-intrusive to the experience.

Variants:

Simple (+) and (-) buttons visible on the screen.

The simple button set that disappears from the UI if the image is not being engaged.

Magnifying glass icons with a + or - in them to help with discoverability issues.

Known Uses:

Seadragon Ajax uses simple buttons that disappear as well as a button to return you to "home".
Google Maps uses the simple (+/-) button set that brackets a UI Slider.
Bing Maps uses magnifying glass variants that bracket a UI Slider.