I bite my fingernails and the skin around the edges of my fingernails. It has stopped me from ever having nice, long manicured nails, but honestly I don't really care. I do it when I'm not thinking, so like when I'm reading manga or watching videos. I've long since stopped biting my nails down to the quick, but sometimes I bite too much skin off and my fingertips are angry at me the day afterward!
Also, this is really random, but I noticed if I am thinking of something embarrassing that I did from a long time ago, I will make a noise -- any noise -- to distract myself. At least, I think it's to distract myself? I dunno, I just do it.