female ichneumonid wasp
likely Netelia sp.
Eh, you can tell I studied entomology 20 years ago by the names I still prefer to use. Apparently, it's now fashionable to call these wasps ichneumons, rather than ichneumonids, as you will find on Wikipedia and BugGuide linked in the ID above. Powell and Hogue state there are probably more than 1000 spp. of ichneumonid wasps in CA. I've always loved the delicate grace of these wasps. These and braconids were featured heavily in my Economic Entomology class, because they are easy examples of beneficial insects that parasitize other insects in a most spectacular fashion. Unfortunately, see that not-insignificant ovipositor? Well, that apparently can deliver quite the sting. A look-alike ichneumonid wasp that does not sting is Ophion spp.