I usually leave $2 with a quick note that simply says "Thanks". I've left more if I leave the place a mess that is bigger than everyday use.
I see no harm in making someones day for a job well done.

An interesting side note: We stayed in a condo in Cabo San Lucas that had daily maid service. I had 2 10 peso coins (roughly $1 each) fall out of my pocket when changing. I left a $2 tip and when I returned the $2 was gone and the 20 peso's was on the kitchen counter. I was impressed.