Tickling as a fetish and especially feet tickling is perhaps one of the most common human behaviors.
studies indicate nearly 85 percent of adults in some way or another enjoy tickling others, being tickled themselves, or watching others be tickled.
Using tools can intensify the tickling sensation, inducing heavier laughter. The tools appropriate for tickling use varies by body part. 
For tickling the soles of feet, using something with sharp points to stroke the soles of the feet can produce intense sensations.
"feet tickling" is a sub part of "erotic tickling" (frequent breaks, safewords, sensual movements), using fingers to tickle feet, 
using the tongue to lick the soles of the feet and toes or items such as feathers and brushes to produce tickling sensations as part of erotic foreplay
Foot tickling is usually harmless and can be done with or without restraint.

Horny Tickling

Tied and Tickled

Tickling Teenies

Ticklish Spots

Tickle Assault