Awesome new plugin which can be downloaded free at this link:
http://www.asuni.com/?p=194
It adds two new features. First is a virtual symmetry, mirroring all objects across any user-defined axis. It doesn't add objects to your scene, just mirrors them in the viewport. Mirror will update as you create or edit any of the geometry. Mirror can be toggled on/off with a button or hotkey...
Second feature is an improved feature like Rhino's clipping plane but this uses any solid volume (surface, polysurface, or mesh) as the clipping object. Essentially it works as a live boolean to visualize cutouts in your objects. But it's for display purposes only, doesn't affect the real object...
shorty