Dogs and cats
Cayenne Pepper is really cheap at Walmart. Buy it in Bulk. Apply liberally. Reapply after it rains. It's nice b/c the birds don't taste it (really!) so it just keeps the varmints out of your yard.
I also keep a supply of bags next to my door. When I see someone letting their dog crap in my yard I run out with a bag, and an enthusiastic, "Oh, you must have forgotten your bag. Have one of mine!"
If I could only remember to THINK about what I was doing before I did it.