Which ruby app is so compelling that people who don't even intend on 
programming ruby are willing to install it?

For Python, there's SCons and a few others.

What are some possible apps we can create in Ruby that will be OUR 
community's killer app?

A modern ViewCVS perhaps?  A cross-platform package manager that unites 
win32, linux and *bsd?  What is realistic, much-needed and compelling 
that we can produce within 12 months?