This attractive young lady is an Eastern Garter Snake (Thamnophis sirtalis). She lives in and amongst the junk that I have laying around the greenhouse. In this photo she's laying on top of a pile of debris left from me sifting potting mix ingredients. This seems to be a good spot to warm up for the day... it's near the shelter of junk piles and brush, but open to the sun.
Later on I caught her basking on top of a rotting bale of straw:
Having garter snakes in the garden is a mixed blessing. One one hand, they eat slugs and insects, but they also love to eat frogs and toads. Considering how many amphibians I see hopping around, I have a feeling that they make up a pretty large part of this particular snake's diet.
I'm pretty sure she's a female. I've caught her before, and her size and shorter tail bring me to that conclusion. She doesn't bite, but she will poop all over you while emptying her scent glands. Lesson: don't pick up snakes unless you're prepared to smell funny.