Over the years I have found it convenient to have quick online access to some tools that help me producing print or web design projects.
Paper calculators are an invaluable printer's tool for many situations and these handy Javascript calculators list the standard paper sizes and weights and have built-in functions with formulas to help you quickly do the math.
Web-safe color charts are a great reference tool to have accessible when you're designing web sites. Even though modern display monitors are capable of displaying millions of colors, it's still a good idea to paint your pages with the 256 web-safe colors if possible, because there are still many older monitors out there.