rvm (ruby version manager) is a kick ass tool that I use on a daily basis. It allows you to install lots of ruby versions, gems, gem sets, and what have you. You can test your app against new (or old) versions of ruby easily. You can also use it to manage rubies on a production system, for all users of that system.
Yes, you heard me correctly. After needling Wayne into getting root support built into RVM so many weeks ago, this is really the only way that I use it. There are a few gotchas however. I will go through my best practices for installing rvm system wide in this article.