Parametric Button Generator with lots of options. This edition features ligne measurements for button diameter. If you prefer metric, or aren't sure what a ligne is, head over to http://www.thingiverse.com/thing:1729283 to generate buttons with the diameter in milimeters.
While the ligne isn't so common these days, it is still used in the garment industry to measure button sizes. For home use, the metric edition is probably your best bet, but I wanted to include the ligne edition for anyone more comfortable with standard ligne button sizing.