HiTechnic, one of the most active providers of 3rd party sensors for LEGO® MINDSTORMS, has published two new sensors: the Angle and the Magnetic sensor.
Similar to the rotation sensor built into the NXT servo motors, the angle sensor measures axle rotation position and rotation speed; while the rotation sensor is located on the motor's axle, though, the angle sensor can be directly attached to the axle in question, hence does not suffer from inaccuracies that arise from friction or gear slopes. It thus is apt to be more reliable on models where this is a critical factor.
Another very nice feature is that it keeps measuring the angle even when unplugged or not powered; so there's no need for re-calibration between power cycles.
The magnetic sensor is a single-axis magnetometer and is able to detect and measure magnetic fields that exist around the sensor.