These are tools we use every day as developers. If you're a Mac-owning web developer, you should consider these apps for your toolkit.
Adobe Shadow - Development tool
Wirelessly connect multiple iOS & Android devices to your computer then synchronously browse, inspect & debug on devices.
ColorSnapper - Color Picker
A global, hotkey accessible color picker for OSX with magnifying loupe. Easily grab hex values for anything.
LiveReload - Development tool
Never hit refresh again. As soon as you save a file, it is preprocessed as needed, and the browser is refreshed.
Patterns - Regular expression editor
Regular expressions made easy with this editor. See matches and replacements in real time while you edit your pattern.
Pixelmator - Image editor
The first solid replacement for Photoshop. Beautifully designed, easy-to-use, fast, and powerful image editing.
TextMate - Text editor
The gold standard for editing code. Syntax highlighting plus extensibility via Bundles makes TextMate the best coding experience on any platform.
Transmit - FTP client
The closest thing you'll ever have to fun in an FTP client. It's elegant, flexible, and easy to use.