A set of 4 bars that go from the rear crossmember on either side of the rear differential to the rear wheel hubs. There are two on each side and the rearmost ones are how rear toe is adjusted.

 

They're stock parts. You can of course upgrade them to something better that might be adjustable and/or have better bushings. But this would be one of the last suspension upgrades IMO. Coilovers, swaybars, endlinks will make more of a noticeable difference, especially for the $.