I start any project with a revision control system. On projects of any complexity, I know you absolutely need the ability to make notes about what you’ve changed every time you make an improvement. And you need the ability to roll back to an earlier version when your improvements weren’t improvements. You also get the bonus of off-site backup of your code (assuming you use one of the many no-cost or low-cost repository hosting services).
A few years ago, the only alternative to proprietary revision control systems was the open source CVS. On occasions, it could make grown-ups cry. Subversion (SVN) has become more popular recently and it’s widely used on both open source and closed projects.
If I’m doing a project just for myself or my minions (that is, with a single point of ultimate authority), svn is the best choice. There’s nothing extra needed when you want to use svn on Mac OS X version 10.5 (Leopard); it is included when you install the Developer Tools package. You’ll just need to sign up for a repository hosting service.
For an open source project, there are good reasons to prefer Git, as detailed here by Dr Nic Williams. Git is the distributed revision control system created by Linus Torvalds (the creator of Linux). The most popular choice for a Git repository hosting service is GitHub.