In case you've been living under a rock, let me clue you in: Harry Potter and the Half-Blood Prince, the long awaited sixth book in the Harrry Potter Series comes out Today (and PS, you've got dust on your cheek).
So who exactly is this half-blood prince??The most popular theories going (since JK debunked the Tom Riddle and Harry himself theories) are Godric Gryffindor or a totally new and never-before-seen character (possibly come to Hogwarts to fill the misbegotten position of Defense Against the Dark Arts teacher). Filling the balance are folks who believe that it's Neville Longbottom (the other half of the prophecy), Seamus Finnegan (sorry, folks, but I think he's a Red Shirt – not that I think he's going to die, but I do think he's just filler), Sirius Black (I eliminate him, because I think he's a pure-blood), and Hagrid (well he is half wizard, half giant).
For my money, I'm betting on an unknown, I think. Though having written this down now, I will inevitably be proven wrong. 
Until you get around to finding out, here's some stuff to chew on:
General HBP Predictions
Princes and Patterns
On Blood and Princes
The 'True' Idendity of the Half-Blood Prince
I want to know who the Half Blood Prince without reading the whole thing. I know, I'm cheating but I'm looking for a spoiler online. Oh, and I'm still laughing at your “You've got dust on your cheek”.