Carnegie Mellon's SensorFly inlcudes a full IMU and an ultrasonic sensor in just 29 grams. Designed for interior autonomy and swarming. Impressive!
"The SensorFly is a novel low-cost controlled-mobile aerial sensor networking platform. A flock of these 29g autonomous helicopter nodes with communication, ranging and collaborative path determination capabilities, can be extremely useful in sensing survivors after disasters or adversaries in urban combat scenarios."