How do you hang things on brick walls and stone fireplaces?

I have a stone fireplace in my room and would really like to hang a canvas up on it, but I'm concerned about putting nail holes into the grout (or whatever that thing is called that is in between the stones) and how I'd repair it if I made a mistake. I'd also like to put some art up on an exposed brick wall in a guest room. How can I safely hang things on these surfaces?