This is my trusty ping pong paddle (or table tennis bat to some).
It is one of my all-time favorites. It is thin and gives a good sting but doesn't leave nasty bruises. It is very good for over the knee spanking because it's compact. It can also cover most of a cheek.
I also like these because they are pretty easily available. Lots of people have these hanging around and it doesn't scream 'spanking implement'. Well it does for people like me :) but not so much for the vanilla world. They are also pretty cheap and easy to find in walmart, kmarts etc when you're in a pinch.
There is just something special about an implement that is found in many homes and lives a non-spanking life until grabbed by a determined discipliner. Just screams domestic discipline to me. Think of how many spankers over the years who have grabbed one from the 'rumpus room' to deal with a bad attitude or saucy mouth. I use it a lot, especially early in a spanking.
It can also leave some interesting patterns on the bottom due to the rubber covering. The bumpy feel of it rubbed across the flesh prior to a smack can really wake up the spankee and build anticipation :)