Eumens fraternus (Potter Wasp).
Size: approx. 15-20 mm. Photo: 08/02/11, 10 am.

This common Potter Wasp (Eumenes fraternus) builds small spherical mud nests (see below, 24 h later). They provision the nest with a small caterpillar etc. after laying a single egg. Also the prepupal instar overwinters in the mud pots built by the last generation of females. 

bug guide (this photo):