There are so many useful things for developing… here are some of my favourites:

• Notepad++  is awesome, what would we do without syntax highlighting?
• Firebug for Firefox is part of me now, I feel handicapped with out it.
• CakePHP, haven’t learned it to well yet, but my co-workers love it.
• jQuery, the magic wand of javascript.
• Better Font Stacks, a great article about… well, font stacks.
• w3schools, a goto, the bible of code

And design aids:

• Typechart, great previews of what is capable with web fonts.
• ColorSchemer is full of pretty and useful schemes and great palette software.
• A List Apart kind of applies to both categories, heaps of good articles