Most Mac lovers love the Mac for the carefully wrought user interfaces and the crisp design, and never pay attention to the open source at the heart of the operating system. But underneath this beautiful facade is a heart built upon the rich -- if often chaotic -- world of open source software.

If you want to go through the pain and joy of building the OS yourself from scratch, you can even download the open source core of Mac OS X known as Darwin.

[ See the slideshow summary of the 10 best open source apps for Mac OS X. Read about the winners of InfoWorld's 2009 Best of Open Source Software Awards. ]

That's just the foundation. There are thousands of open source tools available for the Mac, some built for the Mac alone and others that are translations of software created for other operating systems. Some are aimed at a niche of programmers or scientists, but a good number are supremely useful tools for everyone.

This list includes just 10 of the most essential open source applications for a Mac, all precompiled, polished, and ready to run.

Downloading the software is just the beginning because many of them have yet another layer of openness hidden inside. Several of the applications have their own built-in environment for extending the software. Some accept plug-ins, some have pop-up windows for writing short extensions, and some have both -- so you have even more options for customization.

In many cases, you're not just getting an open source tool; you're getting a range of options to add to that tool.