Fic rec: "Twelve" Ooh, spanking smut! "
Twelve", by Isis, is complete and total Snape-punishing-Harry smut--and it's utterly delicious. Definitely read it: it's delightfully steamy.
Edited to add: It can also be found
here, on Isis's website; the link above is to the Pornish Pixies post where I found it. :D