There are several companies selling rare earth magnet balls, or neodymium balls. You've probably heard of a few of them already: Buckyballs (from whose site the photo to the left came from), NeoCube, ZenMagnets, CyberCube and more. I bought my husband a set of NeoCubes a few years back (the only ones I could find in Canada at the time).
With most of the sets you get 200+ mini balls that you can put together in any number of designs. They're quite amusing and will occupy a geeks and non-geeks alike for many, many hours.
One thing to be aware of, the companies who make these have designed them all to be slightly different sizes from each other. Even within manufacturers your balls could be differently sized (for example, if you get CyberCube's 7x7 green box the balls are 4.76mm, if you get the 6x6 blue box, they're 6mm).
So, though NeoCubes were the only ones available in Canada back when I bought them, I can't find them here anymore, but also can't supplement them with a set from another company either. So take care when deciding which company to go with.
If you're not into balls, here's a site that sells rare-earth magnets of different shapes and sizes, MagCraft.
Seriously though, magnets are a lot of fun. Just play safely. Each site contains a warning against letting children get their hands on them (swallowing magnets can lead to injury or death). You can buy them from their respective sites, a lot of game/puzzle sites online and science stores.
Here's a video from MRJeffsoable's youtube page, showing some of the shapes you can make: