The 555 timer chip has been around since the 70s, so does the world really need another website for calculating the circuit values?
No! But I made one anyway … it's largely an excuse to play around with HTML5 canvas and coffeescript! All the source code for this page is on GitHub.
For an example of a 555 circuit hooked up with Arduino and Processing, check out AstableOscillator and Monostable, from the LittleArduinoProjects collection.