App note from Silicon Labs on their hall-effect Si72xx magnetic sensor to sense position, count revolution and security system seal tamper sensing. Link here (PDF)
These devices all measure magnetic field in the axis perpendicular to the package. All of these parts share the attributes of low power, high sensitivity, and low noise. The simplest parts in this family have a single output pin that goes high or low at a certain magnetic field. Options are available to output the magnetic field by I2C, SENT, PWM or analog format, to put the part in a very low power sleep state (disable pin), to turn on an on-chip test coil (BIST) and to indicate when a higher than expected field has been detected (tamper).