I've written a stand-alone voltage divider calculator in Java, which I have released (including heavily commented source code) under a Creative Commons license. Currently, it only shows results for using 1 Percent resistors; anyone wanting to expand/modify/enhance it only needs the free Netbeans IDE from Sun: http://netbeans.org/.
The program is very simple to use, will copy results to the users system clipboard at the click of a button, and though developed on a Linux box running Redhat 11, it should run on anything that supports Java.
Does it take into consideration what the input impedance of what is wired to the divided voltage point?
No, it doesn't. The Atmel datasheet doesn't give a specific input impedance for either the analog inputs, or even the digital pins; all they claim (that I could find) is "high impedance" -- which could mean anything.
Well in cases where the divider junction is being wired directly to a Arduino digital or analog input pin then the results of your calculation tool should be fine as the 'high impedance' would be high enough not to effect the results except for maybe the most highest resistor choices available (megohms).
However I've seen several beginners to electronics rely on these kind of calculation tools without understanding the fundamentals involved. For instance using a voltage divider to power a 3.3volt device from +5v where the device may need to draw several milliamps of power, and not understand why the voltage divider is not providing the correct voltage.
The input impedance actually is in the datasheet, page 310 on the 168 datasheet to be exact. 100Mohms for analog inputs. Didn't see input impedance for digital inputs but the input current is 1uA.