Disclaimer: Here is a complete answer taking the last version of OS X (10.9 AKA Mavericks) into account. I am aware that everything I compiled in this answer is already present in the page, but having it clearly in one answer makes it a lot clearer.
First of all, with previous versions of OS X, Maven is installed by default. If Java is missing running [email protected]:~ $ java in a terminal will prompt you for the Java installation.
With Mac OS X 10.9 (Mavericks), Maven is not installed by default anymore. Different options are then possible:
Using Macports: (I did not test this)
Maven Install on Mac OS X