Unabomber's - "Strut Bars
Sometimes referred to as strut tower bars.
The technical reason strut bars are desired is that vertical loads from the wheel pass through the strut and spring up to the strut top and towers, which leads to flex in the sheet metal around the strut tower and inner guard. Chassis flex is not conducive to firm response in a vehicle. Installation of a strut bar between the front and/or rear strut towers eliminates this flex. The result is faster and firmer response to steering inputs. Most users report this to be a very minor suspension modification ranked further down on the suspension modification path. On the other hand, the proliferation of inexpensive replica/eBay bars and aesthetically pleasing strut bars make this a common modification.
Wagon owners: Whiteline makes a specific rear strut bar with a quick release coupling that allows easy removal for times when you want to utilize the extra space that folding down the rear seats provides.
Whiteline article on strut bars
Strut bar manufacturers:
Though too numerous to fully list, here are the major manufacturers: