Library for I2C sensors and some atmel-specific functions. The following sensors can be used with an uniform interface and come with arduino-examples:

Austria Microsystems TCS3772: light sensor - RGB and clear
Silicon Labs SI7021: humidity sensor
Invensense MPU9250: 9DOF - 3 axis acceleration and gyro PLUS AK8963-IC with magnetic-field sensor
Freescale MPL3115A2: pressure
Maxim MAX44009: ambient and lux with incredible wide dynamic
NXP PCF2127: Realtime-Clock with 2ppm
Bosch BMP280: pressure
ST L3G-Series: 3 axis gyro / angular rate
Freescale MAG3110: 3 axis Compass / Magnetic field
Freescale MMA8451: 3 axis acceleration
Fairchild FAN5421: Single-Cell Li-Ion Switching Charger
STM LPS331: Pressure Sensor
Maxim MAX17047: Fuel Gauge for various Cells

Additional Features:

AVR: measure VCC of the power-pin
AVR: emulate a spektrum-serial control
fast math fn for e-function, power-of with scaling (interpolation with sampling points)
HDLC: a protocol for serial communication featuring frames, checksum and auto-escaping of characters
WS2812B: efficient controller