Using the middle finger perhaps?
RedLight is what I’ve have my partners use but simply breaking character usually works just as well.
Chicken.
Doesn’t work in full bondage though.
Well, when I was not owned, and playing in public, I tended to go with the general stoplight series:
Red=Stop immediately
Yellow=getting a little much/need physical adjustment (ropes too tight, knees giving out, etc.)
Green=all good
When I played privately, I tended to do what matt_mcl does and address the dom by name instead of “sir” (or their chosen, you get the idea.) I also generally let any partner I was playing with know that if I said “ow” or “ouch” or “stop” that I MEANT it - those are not words/noises I make regularly in any scene.
Himself & I don’t need safewords as much, usually, mostly because we know each other so well, but he’ll respond to any of the above as a sign that something isn’t right. And he has permission to ignore a yellow after questioning me (if it’s “that toy is getting too intense,” he might continue after finding out what the problem is.)
The only time this was a concern, there was no safeword agreed upon ahead of time, so I just had to break character. Yes, it did ruin the scene, but that’s what happens sometimes. Perhaps I should think of a safeword just in case. A friend of mine and her SO use “coffee,” which I only found out because of a fit of giggles on her part.
Monarch Butterfly
It was just “butterfly” until we found there was both a clamp and a vibrator called that.
coconuts. why you ask? my girlfriend claimed if she said that word it would prevent her from sneezing. :dubious:
"Rats"
The reason we chose that particular word is lost in the mists of time, but it probably had something to do with the fact that I hate rodents. Ball gags aren’t a problem, as we don’t use them. If I’m going to be gagged, we usually go for the scarf, but we rarely use a gag. He likes to hear me, and I prefer blindfolds to gags.
I don’t think I’ve actually used the safe word in ages. After 7 years of this, my husband can usually tell by my tone and how I hold my body if there’s a problem, so there’s no need to stop him.
We don’t have an agreed upon safeword, but, then again, we’ve never needed one. Really, I think if we talked it out, it would just be an incentive (ie, can he bring me to the point where I have to say it?).
In a pinch, I would say his name a few times or, if that failed, “Poodle.” (Anyone out there get the reference? :))
The sub’s first name, plus ‘safeword’ works well as you just don’t forget your first name, and sub’s don’t need real names 
In years past, we used “Jingle Bells” - useful as I can’t really think of any scene where you’d intentionally say that, and if gagged, you can still moan/hum/whatever the tune.
Somehow, by extension, “Dashing through the snow” turned up once as a “yellow”
Ralik.
It means… uh… it means… well, I can’t remember now. My fiance knows what it means. I just know it sticks in my head for some reason. 
“stop! no, really!! i mean it this time!!! RUSSELL!!!” just like case sensitive, i think we need to work on ours, too.
my life may be crashing down on me, but i can’t help but giggle at the picture of gotpasswords all stuffed up and frantically humming “Jingle Bells” when his SO does somethings really objectionable… thanks. 
ours is: FLÜGGÅɘNK∂€ČHIŒß∅L∫ÊN
Bonus points for anyone who can figure out the reference
I’m with Seven, “safeword” seems to be the most unambiguous safeword of all.
That kind of reminds me of that one in EuroTrip (a movie which really sucked, by the way).
Yeah, thats the reference. And the movie did not suck. Greatest Cameo ever!
Yeah, but that particular scene was set in the Netherlands so you have to give him credit for being a homer. Besides, the girl was kind of cute. It was a dumb flick, though.
Kinda cute? It’s XENA!
OK, she’s not that hot but I still got a kick seeing her “administer the Testicle clamps!”