CSS Colours presents all the standard CSS colours in nice large swatches. Mouse over the swatch for hex and RGB values.
Colors provides much nicer versions of a subset of the default CSS colours. They’re a bit less saturated, the black isn’t pure; generally it’s more like something you’d actually use in a design. You can grab the values from the page itself, or grab the stylesheets and source files for a variety of CSS preprocessors from github.
David Kadavy writes up tips to point the less the aesthetically skilled on the road to an attractive app or web page. If you want more from David, he has a book and an email course called “Design for Hackers”.
Grid covers the absolute minimum you need to know to get started with creating responsive web pages. If you stopped here, you’d still be creating layouts that work great on a variety of devices.