From BlenderWiki
Mirror
Mode: Object and Edit modes
Hotkey: CtrlM
Menu: Object/Mesh/etc. → Mirror
Description
Mirroring objects flips them across an axis, determined by the Pivot Point which is essentially a scaling of -1.
Use the shortcut CtrlM, then press X, Y, or Z, to mirror on that axis. You can alternatively hold the MMB 
to interactively mirror the object by moving the mouse in the direction of the mirror axis.
See the rest of the Manipulation in 3D space section for additional options available within the transformation modes.
