The BMP085 is a basic sensor that is designed specifically for measuring barometric pressure (it also does temperature measurement on the side to help). It's one of the few sensors that does this measurement, and its fairly low cost so you'll see it used a lot. You may be wondering why someone would want to measure atmospheric pressure, but its actually really useful for two things. One is to measure altitude. As we travel from below sea level to a high mountain, the air pressure decreases. That means that if we measure the pressure we can determine our altitude - handy when we don't want the expense or size of a GPS unit. Secondly, atmospheric pressure can be used as a predictor of weather which is why weather-casters often talk about "pressure systems"

Specifications

• Pressure sensing range: 300-1100 hPa (9000m to -500m above sea level)
• Up to 0.03hPa / 0.25m resolution
• -40 to +85°C operational range, +-2°C temperature accuracy
• 2-pin i2c interface on chip
• V1 of the breakout uses 3.3V power and logic level only
• V2 of the breakout uses 3.3-5V power and logic level for more flexible usage
  
• Datasheet

You can pick up a BMP085 breakout board from the Adafruit shop!