E.T.
I have a neighbor who will babysit our son every once in a while, and yes, I will occasionally buy her a little something, even if it's a $10 Target GC. This year, I gave her son a birthday present. We don't buy for each other's kids, so this was unexpected for her, but I told her it was just a small way for me to show her gratitude for how she helps us out.
I also make sure I write her a Thank You note and I tend to include a picture of my son, just to make it extra special b/c she adores my son.
I have friends who babysit my son too, but we just trade off babysitting for eachother, so that's a wash.