My favorite one for hiking or similar is a random one-off from Sierra Trading Post, but I have another one from Columbia (via REI) that i like almost as much. I usually wear a tank under so I can unbutton it and flap around to get some air, but it's vented 18 different ways so I rarely feel stuffy in it. Mine is a few years old - link below is the most similar one I saw in a quick search.
For situations where I don't want the extra flappy fabric - I have "cooling" arm sleeves, which actually do make my skin feel cool when the breeze blows (or if I'm biking and thus making my own breeze). would still leave your shoulders exposed in a tank, but I've seen people out and about with almost a bolero version of something like this, so that might be something you could hunt down.