How do I convert between the various measurements?
Common U.S. Volume Measurements:


1 tbsp = 3 tsp
1 fl oz = 2 tbsp
1 cup = 8 oz
1 pint = 2 cups
1 quart = 4 cups
1 gallon = 4 quarts


Common U.S. to Metric Conversions:


1 oz ~ 30 mL
1 cup ~ 237 mL
1 quart ~ 1 L
1 oz = 28.4 g
1 lbs = 0.45 kg


Common Metric to U.S. Conversions:


1 kg = 2.2 lbs


Online Converters:


Google: Type a conversion such as "1 tbsp in mL" and it will automatically convert it for you. This is generally the fastest option.
Wolfram Alpha: Same idea, gives you a little more information and lets you disambiguate between similar measurements (e.g. US tablespoons vs. UK tablespoons).
Convert-Me - Cooking Ingredients: Converts to every well-known measurement at the same time. Can convert (approximate) volume to weight and vice versa, for many ingredients.
Food.com Measurement Converter: Same thing, a little easier to use, but slower.


iPhone/iPad:


Kitchen Calculator
ConvertBot
Smart Chef Conversions
Cookulator
Convert
...or just search the App Store.


Android:


Unit Converter Ultimate
Cooking Conversions
...or just search the Play Store.


Printable Conversion Charts


Cooking Measurement Equivalents (InfoPlease)
Measurement and Conversion Charts (StartCooking)
Conversion Tables (Delia Online)
Converting Recipes to Metric Measures (The Metric Kitchen)
Heirloom Weights &amp; Measures (About.com) - archaic/non-standard measurements


(Note: Use your browser's "print selection" feature to print just the conversion chart)