I just got back from NCTM (more on that later), and had a short opportunity to cruise the exhibition floor. At a regional NCTM conference last year I bought some Geofix shapes for my kids, which they and all their friends have loved ever since. I was hoping to get some more, but the Geofix folks weren't exhibiting this time around. (All my mom-friends who requested Geofix will be disappointed, sorry!)
However, I found Magformers, which I think I like even better. They're a bit pricey, but so far worth every penny. I have 5 kids ranging from 9 to 3, and we've all spent the last 3 days spending over an hour per sitting building awesome models. So far we've mostly made models shown on the Magformers website, including:
My three-year old came up with this one on her own:
I need to order some more triangles now...
Anyone can design anything in 3D! http://www.3dvinci.net/