Hakim El Hattab came up with an idea that is so simple and effective, its surprising that its not standard practice, yet. Ladda is his UI concept that puts the loading indicator of an action or submission in the button or field that triggered it. Fantastic!
CSS
Logos In Pure CSS
CSSmatic – The most complete CSS tools for web designers
There are been many nifty CSS helper tools, but this one stands out as a free, web-based option that looks beautiful and is functional. I love that you can get SASS (SCSS) output instead of plain CSS. Check it out at http://www.cssmatic.com/