Before web video was a “thing,” “Dr. Horrible’s Sing-Along Blog” made it that way. It's kind of boggling that it was released in 2008, so much earlier than any other web series phenomenon out there now. (Actually, has anything been BIGGER than "Dr. Horrible" yet? I don't think so.)
The show is a three-episode musical starring Neil Patrick Harris, Nathan Fillion and me, set in an alt-superhero world. It won an Emmy in 2009 and was probably one of the best experiences in my life. The writers (all related, headed by the legendary and mythical Joss Whedon) created an amazing world out of almost nothing, except their vivid and exceptional imaginations. It was a fantastic low-budget passion project for everyone involved, and I am so lucky to have been a part of it.
You can get it on DVD (with a HILARIOUS commentary track that makes up a full OTHER musical in total!!), or on iTunes. One day they may do a sequel. I think they're kind of busy ruling the Marvel universe right now, but you never know what will happen in the future. :)