I'm installing a piece of software that requires the GD1 library as a dependency on my Mac. Searching only turned up old articles that describe multistep processes that include downloading and compiling yourself. That's no longer necessary. As of 2012, the clean, easy way to do the installation is with Homebrew2. Just run:
sudo brew install gd
No muss. No fuss.
GD Graphics Library - The open source code library for the dynamic creation of images by programmers.
Homebrew - The easiest and most flexible way to install the UNIX tools Apple didn't include with OS X. Also, the software that I use now instead of MacPorts. If you ever have to install unix tools, it's the way to go.