Is there a python equivalent of Ruby's 'rvm'?

Yes, it is virtualenv along with virtualenvwrapper.

update: you may install both at once with virtualenv burrito.

Update: the correct answer is now probably pyenv.

For scientific computing, the corresponding tool is anaconda.

Pythonbrew is no longer under development. The former maintainer suggests to use "PyEnv" instead (see below)".

pythonbrew has come! http://github.com/utahta/pythonbrew

pyenv: https://github.com/yyuu/pyenv

I created pyenv which is a fork of Ruby's rbenv and modified for Python. Like pythonz, pyenv also supports Stackless, PyPy, and Jython.