Ninja Ropes Extreme is an online game, where you, the ninja, must cross obstacles by sending out and swinging from ninja ropes.

According Appscout, it was featured in the DVD of the Dr. Horrible sing-along blog,

"About halfway through the commentary, Jed Whedon, Nathan Fillion and Neil Patrick Harris sing an ode to an iPhone Web game called "Ninja Ropes Extreme," which they all played during filming and which brought them all together."

Enjoy!

(From Justin, Dr. Horrible).