Coming off my blog article, "Using the 555 timer as an external clock for the Arduino" -- and suffering through the hassle of figuring out a good set of resistors and capacitors necessary to get the right timing -- I figured I should put together a quick calculator that would do the heavy lifting for me...and here it is!
This calculator will let you specify either how often you want the timer to fire in seconds, or the frequency of the oscillator in hertz. (Yes, one is just the inverse of the other, but I figured this would be easier for a lot of folks). So, just put the value you're looking for and hit the corresponding 'Go' button, and you'll get a list of values that will correspond to a oscillation frequency close to what you are looking for.
For the components, I chose those that come with the Radio Shack 'grab bags' of resistors (part #271-003) and capacitors (part #272-801 & 272-802). There are, of course, a lot more possibilities than these, but for most hobbyists there's a decent chance these will be kicking around.
How often do you want the timer to fire?
What frequency do you want the timer to fire at?
I have included the 'duty cycle' of the configuration. This didn't matter for my purposes -- I just needed a rising signal on a regular interval -- but you might find it useful.
The values above are based on 'perfect' resistors and capacitors. Most common resistors can vary by up to 10% from the stated value, and capacitors by up to 20%. If you find your results differ significantly from what you expect, you should test your components. If possible, prefer a ceramic disc capacitor over an electrolytic (which can be 'leaky') and if precision is really important, look at ordering some resistors with 1% tolerance (and test them anyway!).