If it's left in the room, you get to keep it. If it's a checkout that is.
With money left in a room a guest is staying it, I don't touch it, unless they leave a note that expresses that it is for me. I don't want to accidentally take money that doesn't belong to me. That would cost me my job and any future jobs in the future.
Normally when I get tips its small change. No more than $5. I have at one point received a $10 bill and thought it was the best thing ever. I had no idea what was to come a few weeks later.
One day, I had just finished cleaning a guest's room and moved onto another room. I was working within another guest's room to make sure it was done. He had come (being directed by another housekeeper) and asked me if I had been the one to clean his room
I looked at my board after he gave me the room number and I had been. I remembered his room well. It was one of the cleanest guest rooms I had been in all day. For a moment, I had thought I messed something up and he was coming to ask me to fix it - which I would gladly do.
I nod and he smiles, "I wanted to say thank you." He extends his hand and offers me a bill. At first, I thought it was the gleaming plastic of the blue $5 bill. I tried to deny him the offer but he had already turned and was walking away.
I walked back into my room I was cleaning with the folded bill. And that's when I noticed it wasn't blue. It was green.
A $20 bill. He gave me a $20 bill for cleaning his bathroom. I think I nearly fell over.
That was my first big tip and I only got a few more like that in the future.