Mine is 'Banana', I'll use it when things get too much or I'm uncomfortable, when I'm in pain I say Stop as he'll know it means something's wrong, if you cannot speak, I suggest a taping system, if I have a gag/choked/or hands are tied behind me, if I tap twice very fast on his arm everything stops.
We use a dog training clicker. Very cheap, and usually come with a wrist strap so there's no danger of dropping it! Audible over a lot of noise, attracts attention quickly, and can be used while the mouth is out of action.
One click means "ease off a bit", and repeated clicks mean "stop". Useful for subs who may feel a little awkward about piping up, and feels a little "cooler" than shouting "Kangaroo!" during a scene.
I don't do role play, or bondage but one of my x boyfriends liked it and had a set of cards like referee cards yellow for warning it's getting a bit much and red for stop. He said he had them because he liked to use a gag mask and still neede to convey when things were getting too much.
We use a squeaky ball so it works with a gag. When it's squeaked, I stop everything and ask if I should stop (it's been squeaked by accident a couple times) and if so, I immediately untie/ungag my partner and ask if he needs a drink or anything.
We also have a safeword that's only been used twice because limbs were getting stiff/uncomfortable. Ours is "Toba" in reference to the school trip to Lake Toba where we got to know each other.
We're using colours as safe words, "green" for continue, "yellow" for be cautious but don't stop and "red" for stop.
+1 I use the traffic light system for safe words too :')
I use this one too. I've played at parties, at this system was great with people I haven't played with before, as they couldn't read my signs as well. Yelling yellow, or amber, gives them chance to check I'm okay, make adjustments and carry on without breaking the scene.