The O'Neill cylinder is one of those beautiful retro design concepts for mankind's exploration into space that was concieved in the sixties when the space race was making these kinds of thing look inevitable. The basic concept it to construct a cylinder several miles long at a lagrange point (a point in space where the moon and earth's gravitational fields effectively cancel each other out, providing a stationary position in space). The idea is that these massive structures are rotated around their central axis and the centrifugal force pins everything to the exterior walls, simulating gravity.
The O'Neill Cylinder has a cousin which it is often mistaken for; the Stanford Torus. This is actually a large circular tube which rotates around it's own wheel-like centre to create an artificial gravity effect on the outer wall only which reduces the available surface area for habitation but has the bonus effect of giving the inhabitants a sky.
The Stanford Torus was intended to have an enormous mirror located just outside the station which would be used to reflect light into the habitat on a day/night cycle. Part of me wants to point out how insane these concepts are but to be honest they aren't - they're beautifuly elegant. They only seem insane because of the scale of the engineering effort involved and the assumed costs. Imagine how cool it would be if we actually managed to build one though.