I often go to websites to convert feet to meters or grams to ounces. The appropriately-named Unit Converter does this task offline.
The interface is as basic as it gets: a 3 x 3 grid of blue rectangles for converting everything from Length to Frequency. Inside each is a pull-down menu for choosing the input and output units as well as a box to enter the number you want converted. The answer pops up in a box.
There's a cornucopia of conversions that includes Celsius to Fahrenheit, atmospheres to Pascals and miles-per-hour to kilometers-per-hour. If you've got ounces and need liters, Unit Converter's got your number.