From BlenderWiki

Hot Key: Shift S (edit mode)

[edit] Top Level

• Selection -> Grid - Aligncurrent selection with the closest grid intersection.
• Selection -> Cursor - Move current selection to the location of the 3D cursor.
• Cursor-> Grid - Move 3D cursor to the closest grid intersection.
• Cursor-> Selection - Move 3D cursor to the current selection.
• Selection-> Center - Move the current selection of objects to their geometric center.