I found a critter on my bathroom towel. It's really tiny. Maybe 2 milimetres? I placed it on an envelope for the photograph.
Can someone identify what it is?
That is the larva of a carpet beetle.
Larvae feed on natural fibers and can damage carpets, furniture, clothing and insect collections.
"Anthrenus verbasci - larva side (aka)" by André Karwath aka Aka on Wikimedia
Related question: What kind of caterpillar is this?, though I think that the larva photographed in that example is either of a different species or at a different time period within the larval stage.