Forget the stress balls; if you want fast-acting stress relief, then you want 'Ref' from Jens Dyvik. 'Ref' is a wearable robot that responds to changes in your pulse by nuzzling your skin to calm you down.
'Ref' from Jens Dyvik can also make itself useful when you aren't stressing out. It coaches you on proper breathing techniques by sending "a wavelike touch sensation up the user's arm for an inhale cue, and down the arm for an exhale cue." Even though it looks like an alien from a sci-fi film, I could still see myself wearing 'Ref' during my most hectic days.