The 4' Curve-N-Line Camera Slider from Proaim features straight and curved tracks in a single design. The top and bottom of the slider each have a different track. Set it on one side, and use it for standard, linear slides. Flip it to the other side and achieve curved slides in which the camera maintains focus on a single point while moving along the curved path.
The legs rotate a full 360° to let you turn the slider to the side you need. The legs are adjustable independently, so they can compensate for uneven surfaces. They're also fully removable in case you only plan on using the slider tripod-mounted and want to travel lighter.
The slider carriage runs on four wheels, each with dual ball bearings, allowing for smooth and steady movement as you manually slide your camera. Before flipping the slider to another side, remove the slider carriage by quick release by pulling on a knob, and then secure it to the other side also by quick release.