Is there nothing equivalent to virtualenv for Ruby? It'd be nice to be able to use the system Ruby and just get gem to install into an isolated project specific location.

Check out rbenv https://github.com/sstephenson/rbenv

I've been using rbenv. I didn't realize it could install gems without using sudo when the system Ruby is activated. Thanks!

rvm and its gemsets.

pythonbrew is actually the python equivalent of rvm - https://github.com/utahta/pythonbrew