Knox Bible Online
It will set you back $55 if you buy it new. But you can also find it online at New Advent. You may want to try it online for a few months before deciding to buy it.
If you have a Mac, you can even create a Fluid app to turn the website into a Mac app. (If you want it to remember the last page when you close it, go to Preferences > Behavior > Closing the last browser window only hides the window).