Note: It's very easy to install ctags, I wrote this mostly because when I install things to OS X I like to check google on how other people did it to make sure I'm not missing something OS X specific. I couldn't find anyone else that had commented on this so I did.

The version of ctags that comes with OS X 10.5 (and probably earlier versions) isn't too great. I don't know what the default version is or anything because it doesn't accept the --version switch. Here's how to get a real ctags installed on your mac:

curl -O http://superb-west.dl.sourceforge.net/sourceforge/ctags/ctags-5.7.tar.gz
tar xzvf ctags-5.7.tar.gz
cd ctags-5.7
./configure
make
make install

This puts it in /usr/local/bin which means it avoids the ctags that comes with OS X. This is good in case Apple releases an update for it and squashes your real ctags. To make the good ctags show up first, you need it to be first in your path. If you're using .bash_login add a line like this:
export PATH="/usr/local/bin:$PATH"

Then you can make sure by opening a NEW terminal window and doing: which tags
It should show /usr/local/bin/ctags

Why I use Exuberant Ctags
Although I've bought many text editors (in order: BBEdit, TextMate, Coda), I find Vi(m) is far more useful for most things. It's worth it to learn this powerhouse editor. Specifically, the Tag List vim plugin is why I needed to install ctags.