Various resistors on a PCB

You'll be using resistors in many of your Arduino projects either to protect input pins, control current through an LED, create a voltage divider, make an RC filter, etc. I use all sorts of values from 0.01Ω for current sensing, through 10kΩ and 20kΩ for voltage dividers and up to 82kΩ for connecting opto-isolators to the mains supply.

It's interesting to note that 40-50% of components in circuits are resistors but they represent only about 2-5% of the total cost. With the low cost of resistors, one may not take much notice of tolerances, temperature coefficients and expected lifetime, but rather focus on the more interesting components such as MOSFETs, transformers and the like. However, resistors tend to age and their values change over time with values above 10kΩ generally increasing and values below 10kΩ generally decreasing. If you buy inexpensive resistors or NOS (new old stock) you need to check the actual resistor values before committing them to a circuit. I recently found an old (perhaps 20 years) 1.5MΩ resistor that was reading 5MΩ!

There's an interesting paper on the Vishay website titled Predictable Components: Stability of Thin Film Resistors.