Paintbrush Rack - Magnetic knife holders sold at kitchen-supply stores can be mounted to the wall with a few screws and will keep paintbrush bristles from being squashed. You can also store other metal tools and utensils this way.
49 Brilliant Garage Organization Tips, Ideas and DIY Projects - Page 31 of 49 - DIY & Crafts. This looks like Bouillon Cube jar lids screwed to the board, so jar part can screw onto the lid. I've also seen done with baby food jars.