Eastern Comma: head with black and white spines. Spines on the back of the body usually light colored with darker tips.
Question Mark: body spotted white, with cream to rusty strips.
I've been finding these caterpillars on Stinging Nettles (Utrica dioica). With the spines with the food, it seems like they might be unpleasant to touch, but the spines are harmless. Maybe I've never really looked before, but it seems like there are a lot more than usual (it seemed like there were more of the Comma, and/or Question Mark butterflies in the spring too).