Black and yellow mud dauber, Sceliphron caementarium. I got a picture of one last year, but the lighting wasn't great. Now that I have a muddy pool in my backyard, I can see these guys every single day. But they're still hard to photograph well--very quick. I just saw on BugGuide that they are big spider hunters--bad news with the coming Arachtober. Their abundance may explain my complete lack of Uloborus this year.