Not under the sink, but flat along the bottom of our 24-inch-wide pantry. We found one at a Crate and Barrel outlet store that folds up very thin. We moved up our bottom rollout shelf so that there is just enough space underneath it to slide the stepstool in when it is folded up.

I like it because it is two steps tall, not just one, and it is very wide and sturdy. it is shown in the link...

Here is a link that might be useful: Mini step stool with 2 steps

Thanks I bookmarked that one.. might be what I need! :)