Purchased on Ebay, 2016-04:
1x Tiny RTC I2C Module DS1307 AT24C32 Time Clock AVR ARM PIC SMD Fr Arduino GBNG
$1.08 (no LIR/CR2032 battery)
1x Tiny RTC I2C Module DS1307 AT24C32 Time Clock AVR ARM PIC SMD Fr Arduino S2EG
$0.99 (no LIR/CR2032 battery)
I2C Tiny RTC DS1307 Real Time Clock Module AT24C32 Board For Arduino AVR PIC ARM
$1.60 (with LIR/CR2032 battery)
Industrial High Precision Si7021 Humidity Sensor with I2C Interface Arduino 1PCS
$3.08
1 pcs BOSCH BMP180 I2C Digital Barometric Pressure Sensor Module Barometer Board
$4.30
US-015 Ultrasonic Module Distance Measuring Transducer Sensor DC 5V new
$1.48
Tiny DS3231 AT24C32 I2C Module Precision Real Time Clock Module For Arduino M8
$2.11
1PCs 8 Bits Digital Tube LED Display Module Eight Serial for Arduino 595 Drives
$1.88
For Arduino AVR PIC High Sensitivity Microphone Sound Sensor Detection Module
$1.14
ADXL335 Module 3-axis Analog Output Accelerometer Angular Transducer GY-61
$2.19
DC 5V Smoke LPG Butane Hydrogen Gas Sensor Detector Module MQ-2 New
$1.52