Design historian Steven Heller collects vintage letterheads and shares some examples at Design Observer.
The design of blogs owes much to the letterhead (and, perhaps more obviously, to the newspaper masthead). Blog posts are, after all, public letters "to whom it may concern". The first design I did for Gawker was quite letterheady and I loved & envied my pal Dean Allen's letterhead-inspired design for Cardigan Industries.
Update: Loooots more great letterhead examples at Letterheady. (thx, jenni)