I've just thrown all of them into a museum exhibit. I kind of wish they could all be turned on at the same time. Imagine a whole room-full of those things just beeping and booping to each other...
I sell them. But if they happen to be a certain color, then I put them up against the walls for the feng shui effect to kick in. Also, it generates points for the HHA thing.
I put them on my Alt character in my town so I can collect them, but not clog up my inventory. I love Poltergoids, though, they're my absolute favorite and I keep those in my house.
Before I got the second floor museum expansion I kept them in my drawers.
Now I just toss them in there. I'm planning on saving the ones I'll use to match my rooms in other houses later.
The rest I'll probably end up selling at Re-Tail.