Proportional editing tool

To access, select the icon in 3D window header, or press O. Available in Editmode for all Object types. This tool works like a 'magnet', you select a few vertices and while editing them, the surrounding vertices move proportionally with it. Use the NumPad-plus and NumPad-minus keys to adjust the area of influence.

The proportional editing tool works with translating, rotating or scaling vertices.