If you're like me, you probably say things like "I wonder how the page tables of every process in the system are populated at the moment" a lot each day. Generally it comes down to "do something for each process running, and print it out".
I think my best effort yet is pview.c (with a Makefile to help build the module). It should also act as a good template for anyone wanting to use either debugfs or the seq_file interface with a queue, something which I have previously found lacking. I have found debugfs and seq_file a good combination; much easier to use than relayfs for non-performance critical medium to large output volumes from the kernel.
Just remember to take the right locks before you go poking around inside processes. Obviously this will perturb the system, but it is often handy for those "what if" questions.