Here is a good basic set of tools for being a drupal contributor on OS X

IRC Client:

For more information see the Drupal IRC page.

LimeChat, Adium and Colloquy are really good open source IRC clients.

Local Web Server:

Although OSX does come with its own version of Apache and PHP, the PHP bundle is incomplete (esp the image toolkit) and therefore is frustrating to use with Drupal. Setting it up for development is a hassle, and you'll find it easier to use:

The Acquia Drupal stack installer for Mac and Windows (DevDesktop) configures all the software you need to build a Drupal web site on your Intel Mac running OS 10.5 or later, including Acquia Drupal, Apache, MySQL and multiple PHP versions.
documentation: https://docs.acquia.com/dev-desktop

The MAMP (yes, "the MAMP") has both free and commercial server environments (MAMP and MAMP Pro) that include Apache, MySQL, PHP and PHP opcode caches.

Code Editor:

TextWrangler is a very robust and free text editor. Useful features include native file editing over FTP/SFTP and a terminal command "edit" to open files straight into TextWrangler. Try dragging a folder or multiple files onto the application icon.
documentation: help menu after running application

Dreamweaver is not much fun to use on Drupal PHP code, though it's possible if you stick to code-view-only.

FTP Client

Cyberduck is an open source FTP, SFTP, WebDAV, Cloud Files and Amazon S3 browser for the Mac. You might want to go to your preferences and set TextWrangler as the text editor - when you save a file edited from Cyberduck, Cyberduck will update the server automatically.
documentation: http://trac.cyberduck.ch/wiki/help

File/Directory diff and merge:

KDiff3 is an open source diff/merge tool.
documentation: http://kdiff3.sourceforge.net/doc/index.html

Git:

You will use git for creating or applying patches and eventually submitting modules you develop. Read the git handbook for more info.

There are a few good git apps for OSX, gitbox is minimal and very friendly. Tower has more features but adds complexity to the UI.

Sourcetree is a very robust free GUI git utility for both Windows and Mac. It provides excellent visualization and management tools, but is significantly more complex than the two OSX apps mentioned above.

If you want to try a more advanced editor straight away:
The Eclipse user interface is intimidating at first, and requires a bit of set-up to configure it usefully for PHP/Drupal development. There is a huge amount of documentation for this, but it may be overkill for first-time developers who just want to change a few lines of code.
Eclipse works well on OSX, and can incorporate :

• PHP Syntax highlighting and IDE with PHPeclipse and others.
• Remote file editing (RSE - Remote System Explorer add-on) that avoids the need for a separate FTP client
• Built-in diff, merge, compare between versions, folders and history
• Git integration for version compares and updates.

NetBeans is a mid-weight alternative to Eclipse and can be used in similar capacities. It possesses many of the same features including code highlighting, code hinting, and debugger support. Netbeans also supports git.