Measuring temperature is surprisingly subtle. There are lots of sensors out there; Digikey sells thermistor sensors interchangeable to +- 0.1 C from several vendors for about $3 in onesies. IC sensors tout good accuracy and linearity, and come in both analogue and digital versions for way under a buck. So what's the issue?
The issue is: temperature sensors measure the temperature of the sensor, whereas what we want is the temperature of something else: air, fluid, or some solid object we're trying to control. So the problem is to get the sensor temperature to track the temperature we actually care about. IC sensors are especially bad, because they have stout leads made of copper (400 W/m/K thermal conductivity) and small packages made of plastic (0.1 W/m/K). Thus they basically measure the temperature of their leads, and are horrible at measuring air temperature, for instance.
National Semiconductor used to put out a very useful Temperature Measurements Handbook. Since TI bought them, it seems to have disappeared from the web, so here's the 2007 edition. There are lots of newer IC temperature sensors now, but not much has changed about the properties of plastic and metal, so the discussion in the first few pages about different sensor types and measurement difficulties is still very current.