Really Useful App!
This is already improving my concentration and workflow. Single Application mode doesn't work well for me because I switch between apps a lot and often i have to cut and paste between them or drag and drop between them. In Single Application mode, the other app disappears immediately. This allows the other app to hang around for the number of seconds you select, which gives the opportunity to drag and drop or cut and paste more quickly, with less mousing or keystrokes. Great job!
The only suggestion I'd make is to give users the choice of something like 5 seconds instead of 15 seconds being the lowest that can be chosen.
Very, very useful and well-done app. I'd pay for this on the MAS.