I have a Knuth cheque from back when he sent out real cheques. Or at least I did; for some reason I decided that cashing it was a good idea and so I have his note back to me and a photocopy of the actual cheque.
I have one for $7.68 ($2.56*3). Alas, it was for some minor typos and grammatical errors in SPOCS [1], not "real" math errors in TAOCP. Never plan on cashing it in though. And I can't remember where I stashed it, so there's that too... :)