Jazz up your websites:

Insert placeholder images (also placedog.com)
http://placekitten.com

Fake text (others: hipster ipsum)
http://baconipsum.com

Colours and patterns for backgrounds
http://colourlovers.com

HTML resources
http://diveintohtml5.info/

Amazing articles and stuff
http://smashingmagazine.com

CSS tricks
http://css-tricks.com

Codepen - Online editor that's magical
http://codepen.io

Cool usaged of Media Queries
http://mediaqueri.es/