DanTimer

dantimer.rb

#!/usr/bin/ruby

require 'time'

def format_time(seconds)
  hours = (seconds / 3600).to_i
  minutes = ((seconds % 3600) / 60).to_i
  seconds = (seconds % 60).to_i
  minutes = "0#{minutes}" if minutes < 10
  seconds = "0#{seconds}" if seconds < 10
  "#{hours}:#{minutes}:#{seconds}"
end
start = Time.now
ARGF.each do |line|
  time = Time.now
  puts "#{format_time(time - start)}: #{line}"
end
puts "#{format_time(Time.now - start)}: DONE!"

usage

$ ./dantimer.rb > notes.txt
Start of show
Merlin talks about agency
Construction at Marco's house
Some cool new mac thing
Siracusa insists he hasn't listend to Build & Analyze
Merlin still talking about agency
^D
$ cat notes.txt
0:00:02: Start of show
0:00:10: Merlin talks about agency
0:00:18: Construction at Marco's house
0:00:27: Some cool new mac thing
0:00:40: Siracusa insists he hasn't listend to Build & Analyze
0:00:50: Merlin still talking about agency
0:00:51: DONE!

Sorry, but I couldn't resist the golfing opportunity! (I always learn something..)

start = Time.now.to_i
l = ->_{ puts Time.at(Time.now - start).utc.strftime("%T")[1..-1] + ": " + _ }
ARGF.each(&l)
l['DONE!']

Or saner/more conventional:

start = Time.now.to_i

ARGF.each do |line|
  puts "#{Time.at(Time.now - start).utc.strftime("%-k:%M:%S")}: #{line}"
end
puts "#{Time.at(Time.now - start).utc.strftime("%-k:%M:%S")}: DONE!"