evaporative cooling off your noggin
I checked these out at the local Patagonia store, but ended up going with the mesh trucker hat, as I like the two-tone colors and the texture change with the mesh, as well as the ventilation on that model.
This style is every bit as nice as the mesh one, and you do get better sun and mosquito protection with these. In warm and hot-weather climes, you also get the benefit of the evaporative cooling off your noggin because of the organic cotton cap. You can't go wrong either way...