From BlenderWiki

To Sphere

This command “spherificates” the selected elements, i.e. it moves them along the lines joining them to the current pivot point, aiming to place them on a sphere centered on the pivot point, of which radius is the median distance of the selection to this pivot point.

All this is a bit difficult to explain, see the example below…

Even if it works in Object mode, this tool is mainly useful in Edit mode, especially with meshes.

Example

This tool allows the creation of spheres from subdivided cubes. First, start with a Cube. Let’s start with from fresh by erasing all (CtrlX).

• Press ⇆ Tab to switch into Edit mode.
  

  Subdivide button in Editing context.
• Make sure all the vertices of the cube are selected by pressing A twice. Then, go to the Editing context by pressing F9. You should be able to see the Mesh Tools panel now.
• Subdivide the cube by pressing the Subdivide button in the Mesh Tools panel, or with W » Subdivide. You can do this as many time as you want; the more you subdivide, the smoother your sphere will be.
• Now, press Ctrl⇧ ShiftS and move your mouse left or right to interactively control the proportion of “spherification” (or directly type a value, like “1.000” to achieve the same effect as below) – preferably using the Median Point pivot point!
• Alternatively, you can use the To Sphere button (in the Mesh Tools panel). Select “100” to make your sphere. Note that you should not move the 3D cursor – or you won’t get a sphere, but a piece of sphere…

Finished low-res sphere!