chruby is an ultra-minimal alternative to RVM and rbenv. chruby allows one to install rubies into /usr/local/$ruby, /opt/$ruby or ~/.rubies/$ruby, but install gems into ~/.gem/$ruby/$version. Unlike RVM or rbenv, chruby only modifies $PATH, $GEM_HOME and $GEM_PATH, and does not hook cd or rely on shims. chruby is ~80 lines, supports both bash and zsh, and has unit-tests.