Why do so many gems end in fu?
ruby, ruby-on-rails
Solution
It is, as @meagar said, a play on 'kung-fu'. It's an old, old practice. A few plugins for the GIMP are called ***-fu, and you can hear people say that their vim-fu is strong, to say that someone is particularly skilled with vim.
Problem
Why do so many gems end in fu? Not that this make it clear to me, but here is what Google says "define: fu".