Just brew tap homebrew/apache and then brew install <formula>.
If the formula conflicts with one from mxcl/master or another tap, you can brew install homebrew/apache/<formula>.
You can also install via URL:
brew install https://raw.githubusercontent.com/Homebrew/homebrew-apache/master/<formula>.rb
brew help, man brew, or the Homebrew wiki.
A common problem on both OS X 10.8 Mountain Lion and 10.9 Mavericks systems is an error about a missing OSX10.8.xctoolchain or OSX10.9.xctoolchain directory:
/usr/share/apr-1/build-1/libtool: line 4574: /Applications/Xcode.app/Contents/Developer/Toolchains/OSX10.8.xctoolchain/usr/bin/cc: No such file or directory
This is because the OS X tool apr-1-config returns a path for a compiler that does not exist, even with Xcode installed:
$ apr-1-config --cc
/Applications/Xcode.app/Contents/Developer/Toolchains/OSX10.8.xctoolchain/usr/bin/cc
$ apr-1-config --cpp
/Applications/Xcode.app/Contents/Developer/Toolchains/OSX10.8.xctoolchain/usr/bin/cc -E
$ which apr-1-config
/usr/bin/apr-1-config
The simplest solution is to go to the /Applications/Xcode.app/Contents/Developer/Toolchains/ directory and create a symlink named OSX10.8.xctoolchain or OSX10.9.xctoolchain to XcodeDefault.xctoolchain. This requires you to have Xcode installed. If you only have the Xcode Command Line tools or OSX-GCC-Installer, a simple symlink will not work.
This single-line command will set up the symlink if you have Xcode installed, and if you don't, it will create directories leading up to the toolchain and a symlink to /usr/bin that will satisfy the requirements needed for apr-1-config to find the compiler it needs:
sw_vers -productVersion | grep -E '^10\.[89]' > /dev/null && bash -c "[ -d /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain ] && sudo -u $(ls -ld /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain | awk '{print $3}') bash -c 'ln -vs XcodeDefault.xctoolchain /Applications/Xcode.app/Contents/Developer/Toolchains/OSX$(sw_vers -productVersion | cut -c-4).xctoolchain' || sudo bash -c 'mkdir -vp /Applications/Xcode.app/Contents/Developer/Toolchains/OSX$(sw_vers -productVersion | cut -c-4).xctoolchain/usr && ln -s /usr/bin /Applications/Xcode.app/Contents/Developer/Toolchains/OSX$(sw_vers -productVersion | cut -c-4).xctoolchain/usr/bin'"