Earlier I posted pictures of a Spot-winged Glider (Pantala hymenaea) and Variegated Meadowhawk (Sympetrum corruptum). Recently I've also seen Black Saddlebags (Tramea lacerata) and Common Green Darners (Anax junius). On 12 September 2012, I spotted my first Blue Dasher (Pachydiplax longipennis) on Bodega Head. It's not necessarily a rare species, but I hadn't seen one on Bodega Head before.
On 10 September 2012, I photographed this Twelve-spotted Skimmer (Libellula pulchella) in the dunes.
Note that there are three black spots on each wing — at the base, in the middle, and at the tip. The pale yellow stripes along the side of the abdomen form a continuous line. The short appendages at the tip of the abdomen reveal that this is a female.
Here are two more shots from different angles.
The males also have a total of 12 black spots on their wings, but as they get older they develop prominent pruinose white spots and a pruinose abdomen. (Pruinosity refers to a wax-like coating; it's usually white or blue. You can see the same sort of "bloom" on fruit such as plums or grapes.)
Look for these species in warm, sunny spots that are out of the wind.