The Hokuyo UTM-30LX Laser Rangefinder is a great choice for doing 2D slam with gmapping or similar solutions. It has excellent performance with a 40Hz scan rate, a working range of 0.1 to 30m and a 270° field of view.
If you are installing one on your robot it might be useful to know where the optical center is and where the transmit and receive optical axes are located. So, after having previously taken the Hokuyo UTM-30LX apart, we finally got around to working out some mechanical drawings. Keep in mind that the full cylindrical viewing region should be kept clear of occlusions for best performance.
If you need a CAD drawing, it is now available here in STEP format. (right click, save as, etc) The critical dimensions should be accurate, but there are some minor cosmetic details left out. Caveat emptor, as always when it comes to these things.
If you are looking for the Hokuyo URG-04LX Laser Rangefinder it is over here.