• Watch the movie with the Wiccan subtitles on and after a couple minutes of silly text, there is a message that reads ENTER AT 9780793529865. Google the number and it's the ISBN number for this Mariah Carey book.
  
• Watch the Making of Dr. Horrible Featurette on The Music and press ENTER when Felicia Day mentions Mariah Carey. (If you advance by chapter through it, it's Chapter 3, so press forward twice.) You get a countdown clock. Here is one screencap of what's going on during the countdown.
  
• Watch the end credits for all of the Making of Dr. Horrible Featurettes and certain letters are highlighted. They read enteratact2. So obviously you hit ENTER when you start act 2.
  
• And when you do hit enter at the start of act 2, you get another countdown clock. This one.
  
• I found this one on the web and I'm not sure how they discovered it, but if you start the movie with the scene Moist Dries Up in act 2, you get a third countdown clock. This one.

Now here is where I'm a little bit stumped. I know "Steve's Song" in Commentary! The Musical spouts out this number: 76606.762359 squared, which is about 5868596039. I'm thinking it's a phone number, but I've dialed it and I'm getting a busy signal. In any event, the number is a prime number. No idea of its significance.

Other things I've heard on the web is that pushing 5 during the countdown clock with the Wonderflonium briefcase, you get to an Easter Egg. You also get to the Easter Egg that comes with the countdown of Professor Normal's head by pushing 5. During the countdown you get with Dr. Horrible's head, if you push 6, you get an Easter Egg. Note this doesn't seem to work when you're working on a computer. Or at least it didn't for me. Anyway, it appears that you count the number of times one of the following letters appears in what the countdown's of: U, R, D, and L. These letters are on the DVD label near the words "count the letters." Tricky because the way U, R, D, and L are arranged, it looks like they stand for Up, Right, Down, and Left.