I remember taking part in a FAP thread a couple of years back, about who sent that Valentine (said thread is probably still going strong). JKR should have made it a much crappier poem - the scansion's suspiciously good for an 11-year-old girl's original work. The Harry/Tom shippers like to assume Riddle helped Ginny write it; and Draco is a popular suspect - well, in OotP we even got further confirmation that he can rhyme.

I still say it was Peeves.