Apparently it was a black rat snake
Which are common in this area
After leaving randomly hysterical messages for Mark and Catherine, I went across the street to Al's. Al turned out to be way more afraid of snakes than I was, but the construction guys over there got Josh's attention, and he came over and helped me catch it.
He and Dan were total boys about it, showing off and trying to scare people, and their goofing off allowed the snake to get away. They recaptured it, tho.
They did call animal control, who apparently came and picked up the snake after I had already gone to the swimming pool. I'm glad they didn't hurt it -- I was kind of concerned when Josh first came inside. I think if I'd known at the time that it was a non-venomous snake, I probably just would have encouraged it outside and into the yard all by myself.