Adafruit TMP007 Infrared Thermopile Sensor
Adafruit Contact-less Infrared Thermopile Sensor Breakout - TMP007
Unlike most of the other temperature sensors, this breakout has a really cool IR sensor from TI that can measure the temperature of an object without touching it!
The TMP007 is, and is an update of the TMP006 thermopile sensor from TI. The internal maths engine does all the temperature calculations so its easier to integrate - you can read the die and target temperatures directly over I2C. The TMP007 also has better transient management, so you don't get as much over/undershoot when the temperature changes a lot.
Simply point the sensor towards what you want to measure and it will detect the temperature by absorbing IR waves emitted. The embedded thermopile sensor generates a very very small voltage depending on how much IR there is, and using some maths, that micro voltage can be used to calculate the temperature. It also takes the measurement over an area so it can be handy for determining the average temperature of something.
This sensor comes as a ultra-small 0.5mm pitch BGA, too hard to solder by hand. So Adafruit stuck it on an easy-to-work-with breakout board. The sensor works with 2.5V to 5V logic so it requires no logic level shifting. There are two address pins and using a funky method of connecting the pins you can have up to 8 TMP007's connected to one i2c bus. Also includeded is a small piece of 0.1" breakaway header so you can easily solder to and use this sensor on a breadboard. Two mounting holes make it easy to attach to an enclosure.
Of course, Adafruit wouldn't just hand you a datasheet and wish you luck, they provide a great tutorial + easy-to-use Arduino library with an example that will have you up and running in 5 minutes. The code can also be ported to any microcontroller with i2c support.