I made an addition to my website http://www.mattspinelli.com
by adding an online PPI calculator. You can access the calculator through my home page or go directly to it at http://www.mattspinelli.com/ppicalc.html
The calculator lets you enter either the image size or the number of megapixels and the aspect ratio. It also gives you the option to input a custom print size. It will then calculate the ppi for the some of the most popular as well as larger sized prints. If you entered a custom size it will display this one first.
Please feel free to use it and test it. If you find any bugs please let me know. Thanks!