We have a stack of Oh! Chairs, designed by Karim Rashid for Umbra, in our place in DC. When we needed some stacking chairs, I had the image of it in my mind--left over from an old NYT article about the making of--but couldn't find who or where. Turns out they're available at The Container Store (and were, thus, invisible to me).
The chair comes in various, ever-changing colors; we hunted down a set in translucent orange, but a friend has several matte black ones in his gallery. Right now at the store, they're all opaque: red, white...turquoise...pink....
Anyway, from the moment the kid could stand up, I'd often take a chair down and plop it in the middle of the living room floor. She'd use it for standing practice, play peek-a-boo in all the little kid-level cutouts, and play with toys dumped on its seat. It has been the improvised exersaucer and jungle gym we never had/wanted.
Then, last week, I moved the chair off the carpet to the hardwood floor, and she took off. Now she runs laps with the thing, cruising up and down the living room floor and entry hall, pushing the extremely light, easy-to-slide chair.
One note: the narrowly set rear legs make it easier than you think to tip sideways. It's only happened once to the kid, but still, a heads up.
Buy some Oh! Chairs at The Container Store for $39.99 a piece [containerstore.com]
Love these chairs!
My roommate and I each bought one in college, mine was black his was white - we called them out ying and yang chairs.
They are surprisingly durable too