I'd like to present you with my newest project, a very small application that will help you figure out the chance your spell has to succeed versus the average abilities of the chosen challenge rating. To run it, make sure you have the most recent Java Runtive Environment version.
The GUI (stands for graphical user interface) is really straightforward - you enter the DC of your spell, your caster level and the CR you're testing your spell against; tick the appropriate boxes for your spell (e.g. if you're testing Ray of Enfeeblement you'll only tick the 'SR' box, while against Phantasmal Killer you'll have to tick 'Fort', 'Will' and 'SR'. Finally, press calculate and the bars under the button should be populated with the statistical chances your spell has to succeed - if a saving throw or spell resistance is not ticked, the appropriate bars will have a 'not applicable' indication on them.
Important: Make sure that you close the application using File->Exit or the Ctrl+Q shortcut and not the "X" button.
The average abilities are taken from an old graph I once made for the old Wizards' Handbook, back on 339, using Cubeknight's critter filter:
If you have any problems, suggestions or you want me to implement more features (I plan to add attack rolls, too, totally forgot about them!), just leave me a comment.