While tickling my wife I usually just use my hands because she is rarely tied up. When she is tied I do have a few things that really get to her. Feathers dragged slowly between her toes drives her crazy. They usually do not work on her soles, she needs something a little stiffer. She doesn't like the feeling of her soles being brushed either. She says it hurts more than tickles. MY FAVORITE tool is an old Christmas decoration. It's molded plastic and flat on one side, but the other side is very rough (think 1000 fingernails) and when I drag that along her soles she can't stand it for very long. It gets, by far, the best reaction to foot tickling.
I had a tickle session with Lolo from T/A some time ago and she was reacting to everything I touched her with, however, when I ran those big billowy feathers you see in the videos down and around her neck she couldn't stand it and safeworded almost immediately. She told me that the feathers were pure torture for her. She said she was going to throw those GD things away...lol