Shabadu found this awesome tribute to Dr. Horrible's Sing-a-long Blog.  It's a conceptual animation of an 8-bit game based on the series.  I've hosted a copy on my site (http://darkshade.homeip.net/stuff/drHorrible.swf) in case the original ever disappears (http://www.doctoroctoroc.com/8-bit-dr-horrible/).
You can also download the 8-bit music album as mp3 from the original site.