FriXion Ball Slim - 0.38mm. More FriXion pen colors and options here
About FriXion:
The FriXion ink is erasable by rubbing with the tip of the pen to create the frictional heat needed for the ink to disappear. Since the ink reacts directly to temperature, it is recommended not to leave it under direct sunlight or anything that's hotter than 140 Fahrenheit.