I assumed it was from my dad, because he sent me an online gift certificate for a website that has printer supplies (save $5 on a $50 order - WTF? I still have to pay $45! What kind of gift is that?). Turns out, they don't have supplies for Dell printers, which is what I have. This is why I give him a wish list of the stores I want gift certificate for!!!!! Best Buy was on that list. But it wasn't from him.
My next thought was that it might be from my brother, since he knows I shop there. It also says that the postage was paid for in Minneapolis, where he lives (okay, next to where he lives). But I'm IMing with him now and he said it's not from him. In fact, he asked if he should mail my present or wait and give it to me when I see him at the end of January. I told him to mail it.
So now I have a lovely gift certificate and no one to thank!