DTm counter (http://morphett.info/turing/turing.html)

turing.txt

; This program counts the number of a characters in a string
; and appends it in reverse binary to the end of the tape

; Go to nil state
0 * * * nil

; Counting program
nil   a x r odd
even  a x r odd
odd   a a r even

nil * * r nil
odd * * r odd
even * * r even

; Put down 0 at the end of the tape if the number
; of a characters is even. This is the case if we
; are in the even state.
even  _ 0 * return
odd   _ 1 * return
nil   _ # * halt

; Return to tape start
return _ _ r nil
return * * l return