There are some non-Free (but cheap) applications that are in most cases better than their free counter parts (and well worth the price many times over), but as we are not allowed to post them here, I will stick to the "free" list. I did post a list of useful, yet non-free tools on STG.
For Mac OS X
Text Editors
Smultron
Editra
TextWrangler
JEdit
IDEs (Integrated Development Environment)
Aptana Studio - Has several "modules" such as PHP which you can install.
Zend Studio PDT - fantastic free IDE, missing only a few small features from the $399 ZsE.
Eclipse - Get the Eclipse for PHP Developers.
NetBeans - I've never tried this IDE.
Komodo Edit
MySQL Tools - Both the Navicat and Workbench tools should be used over phpMyAdmin. Actually, once you starting using a tool like Navicat, you will NEVER want to touch phpMyAdmin again, and may want to remove it from your server. Navicat has become another "arm" for me.
Navicat Lite - (OS X, Windows, Linux)
An integrated GUI tool (application) that lets you perform MySQL administration and database development in an easier way.
MySQL Workbench
A visual database design application that can be used to efficiently design, manage and document database schemata.
MySQL GUI Tools - (OS X, Windows, Linux)
Webserver (and environment)
MAMP - I use the "pro" version, but the non-pro version will be more than enough for everybody.
XAMPP for Mac OS X
Subversion Version Control
Subversion Binaries - Install this first. - You can then run ALL SVN commands from Terminal.app
SmartSVN GUI App. (recommended) - Contains DIFF tools.
Subclipse for Eclipse - Merge Client included.
Subcommander GUI App.
svnX GUI App.
SCPlugin - Contextual Menu plugin for the Finder.
Git Version Control
Installing git on OSX - Github guide.
Git OSX Installer
GitX - git GUI for OS X.
DIFF Tools
DiffMerge (recommended)
FileMerge via the Mac Developer Tools. (Free)
FTP/SFTP Clients
Cyberduck - Supports S3 and WebDAV.
FileZilla
RegEx Tools
Reggy - recommended.
RegExhibit - (recommended).
RegExToolbox - widget.
RegExWidget - widget.
Other
PHPfr - PHP Function Reference widget (recommended)
Firebug - Firefox extension.
FirePHP - Firebug extension.
Firebug Lite - For Safari, IE and Opera browsers.