Typetester
Typetester lets you try out up to three fonts side-by-side. This can be a
great way to test font stacks to see what the differences will be as the
stacks degrade. You can use any font on your system, though it does
categorize fonts by web safe, Windows default, and Mac default, making it
easier to create stacks without other reference materials.
Font Stack Builder
Code Style offers the Font Stack Builder for creating font stacks based on
font family, and whether the fonts are generally installed on Windows, Mac
or Linux machines. You can add as many fonts as you want and it will show
you a grid with how common those fonts are on different kinds of
machines.