• Pixels to Ems and back http://pxtoem.com/
• Colour names from hexidecimal http://chir.ag/projects/name-that-color/
• Alternative Lorem http://www.boganipsum.com/
• More animated preloaders than you can shake an hourglass at http://preloaders.net/
• Grey box (or otherwise) placeholder images http://placehold.it/
• CSS3 gradient creator http://www.colorzilla.com/gradient-editor/
• CSS selector independent grid framework http://zengrids.com/
• Nnja CSS to styleguide/documentation library https://github.com/thomasdavis/kaleistyleguide
• Breakpoint mixins for SASS https://github.com/canarymason/breakpoint
• Pantone variables partial for SASS https://github.com/leemarkwood/sass_pantones
• Beautiful icons by Entypo CC BY-SA 3.0 http://www.entypo.com/
• Searchable type icon sets (including Entypo) from Fontello http://fontello.com/
• FInd all the colours in your CSS http://www.colorfyit.com
• Compatibility of web features at Can I Use? http://caniuse.com/
• Compressing PNG graphics
  • As a service http://tinypng.org/ 
  • As a command line tool http://pngquant.org/

WCAG2.0 tools (Accessibility)

• Colour contrast checker http://www.checkmycolours.com