We've already shown you an elaborate IKEA-based cat exercise system, but if you need something more compact, this might be just the trick.
At IKEA Hacker, Cave Lion details a simple cat climbing system that doesn't require much more than a bit of work on the bog-standard $40 Billy shelving system. It's a simple but effective solution that will keep most cats happy and doesn't cost the earth. Hamsters? That's another story.
Cat climbing shelf in one compact Billy [IKEA Hacker]