Apple's latest Mac OSX release, 10.5, dubbed "Leopard", does a great job to remedy the problem Rails developers had with previous releases - Ruby is now installed and works correctly, with Rails and a whole bunch of other common gems installed too. Awesome stuff.
Firefox has Firebug, the best JavaScript debugger on the face of the planet. The developers came up with a subset of Firebug that works on all web browers, creatively named Firebug Lite and should be an essential part of any web developers toolkit.