Found Letterheady as referenced on the minted blog. It features famous letterheads both old and new. It's awesome! I especially love the pixel flicks design, where the different shapes trade places and reorganize to form the words on the last line. And look how much space many of the designs take up, as if they were braver and more confident than lots of designs nowadays are.
Here are a few favorites/shockers:
Letters of Note is the sister site, and features "correspondence deserving of a wider audience." Many "fascinating letters, postcards, telegrams, faxes and memos. Scans/photos where possible." It's definitely worth checking out. Read more in an interview here.