I found this caterpillar while biking through Cambridge. I pulled over, flung my bike down on the sidewalk, and made a car wait for me while I picked it off the pavement in the traffic lane.

I think it's a Virginian Tiger Moth (Spilosoma virginica).

I have been told to be careful about picking up fuzzy caterpillars, on the theory that the hairs are defensive irritants, but I have yet to encounter one potent enough to cause me problems.