It's a simple concept whose flexibility & performing implementations can be hard. Usually takes geniuses to come up with one that works well in concept and practice. It's why Butler Lampson always thought they were a waste of time. I don't. So, here's some examples of successful application. :)

http://www.cl.cam.ac.uk/research/security/ctsrd/cheri.html

https://en.wikipedia.org/wiki/EROS_(microkernel)

Note: Definitely see KeyKOS as it was commercially deployed with both POLA and persistence of app data.

https://www.combex.com/tech/darpaBrowser.html

http://www.cs.washington.edu/homes/levy/capabook/index.html

Note: Especially see System/38 which became AS/400 & IBM i. Still selling. And they run and run and run. Architecture & POLA go a long way there.