It's been several years since my ferrets passed on, but I was quite impressed with just how smart they were (well, really only one of my two... the other was sweet, but wasn't particularly bright).
My male surprised me several times with what appeared to be some fairly clever problem-solving.
For example, he was obsessed with getting into the trash can. To do this, he started by reaching up, hooking his claws into the plastic liner (which he could just barely reach), and then putting his weight on it until the can tipped over (I caught him swinging back and forth while dangling from the liner to tip the can).
I solved this by raising the can up so it was too high for him to reach the top.
He responded by opening the bottom cabinet drawer next to the can, climbing in, then opening the one above, climbing into that one, and on until he reached the top (using them like stairs).
As much as I love my cats, I can't imagine them doing anything quite so involved.
I know intelligence is an incredibly difficult thing to quantify, but have there been any credible studies on ferret intelligence, particularly in the area of problem solving capabilities? How do they rank compared to other common pets?