Nothing gets as tiresome as summertime sweat dripping into your eyes on a long ride. Castelli has a remedy in their Summer Skullcap. It's made to fit nicely underneath your helmet, with an airy mesh construction in the top portion. The main working part of the skullcap is the absorbent headband. It wicks moisture from your skin before it can accumulate and drip into your eyes. Also consider this -- the mesh top acts to block the sun from your scalp. So if you have a helmet, like the Giro Aeon, with big vents and lots of them, you might consider this a nice form of protection if your hair isn't what it used to be.
The Castelli Summer Skullcap is White w/red and comes in one size.