In Batman: Arkham City for the PC, PS3, and Xbox 360, you can perform a slide to access items and areas through small openings.
To slide, you just need to run and press the crouch button while running.
On the Xbox 360, you press and hold A to run, and while running press RT to perform a slide.
On the PS3, you press and hold X to run, and while running press R2 to perform a slide.
