Locktopus

An excellent tool to lock a few applications if you don't want other people to launch them (asks a password). The good thing with this app is that it is very simple to protect or not an application: double click the home button (just like to delete them) and tap the lock. It's not high security though. For instance, if the application is already launched and the iPad is paused, when it resumes you directly go to the application without any password required.

See the reference side-by-side comparison iPad applications