I started a project to take 6mm-scale figures (Epic 40k at this point) and individually base them so I could use them in a skirmish-style game. My goal was to make something like Warhammer 40k, but scaled down to take up less room. And I like really tiny things.
Inspired by a number of projects on the web, I decided to base them on magnets and play on a metal surface to minimize the amount of moving around the pieces would do. I tried a number of different options to see what would work best. Here's a first pass on magnetic bases.
Some of these are based on cut-outs from sheet magnets (like you can get a craft stores...refrigerator magnets). Some are based on rare earth magnets, and those work well. I had originally ordered 6mm magnets (about 1/4", the size of an Epic mini base), but found that you can't bring the bases to close together (the poles will repel each other). I order some 4mm and 2mm magnets to try out different combinations.
What I ended up doing was drilling 2mm holes in the bases and gluing 2mm magnets flush with the bottom. This gave me the look I wanted (a scaled-down 28mm miniature) and enough magnetic pull to adhere well. Here are the beginnings of some groups I've put together.
These should work will with square-gridded maps too. Here's some miniatures layed out a 1/4" grid map (originally intended for the Star Wars miniatures game, 25% scale). The map is sitting on a small magnetic whiteboard.