Problem:

Create a way to zoom in and out that makes sense to users and provides spatial cues as to current and previous location

Rationale: 

When zooming, zoom such that the point under the cursor remains stable.  It is hoped that the user will always be aware of the location of that cursor as they move it around the screen.  This is a natural with click-to-zoom, but it's also the best choice for scrollwheel zoom, and many of the top zooming sites are adopting this approach.  There can be some discoverability problems, as it is not always clear how the scene is changing.

Variants:

Most implementations simply zoom around the area underneath the mouse.  Google Maps uses bracket indicators to help users discover what area they are zooming around.

Known Uses:

Seadragon Ajax utilizes zoom to point.

Lila.io utilizes zoom to point.

OpenZoom utilizes zoom to point.

Zoomorama additionally uses overlay cursor for zoom to point, mostly for mobile devices users. (Patent pending)

MosaicManiac utilizes zoom to point.

Google Maps utilizes zoom to point

KrPano has ability to use zoom to point.