http://rvm.io (!!! worth having a look there for the most up-to-date installation procedure !!!)
Allows you to work very well with more than one ruby installation, just like App::perlbrew for perl.
The rvm command is implemented through a lot of bash functions.
The PragProg ruby3 and the jruby book both talk about rvm, and it looks, as if rvm is really quite helpful, if you deal with more than one ruby interpreter.
I installed rvm locally on an openSUSE-11.3 system and also remotely (git-based), and also on Mac OS X 10.6.6 (not git-based (yet)).
Update 2011-02-22:
Output from my $HOME/.bashrc disturbed the smooth execution of rvm, I had to remove that output (although I really liked that).
Update 2011-07-15 – how to upgrade:
$ rvm get stable
$ rvm reload
$ rvm list known
$ rvm list
$ rvm install jruby-1.6.99999
$ rvm install ruby-1.9.99999
$ rvm all do gem install xml-simple
$ …
Update 2012-01-… – trouble with “get head”!!!
This really caused me a lot of grief and fear.
$ rvm get stable # better for me
Update 2013-09-30
(Nowadays) before running rvm, you need to run some initialisation code:
source $HOME/.rvm/scripts/rvm