Rachel Andrew has a good article about some of the emerging features in CSS. Most are already available in one or more of the modern browsers so you can test and play with them already.
http://www.webdesignerdepot.com/2015/01/css-you-can-get-excited-about-in-2015/
CSS Tricks have a largeĀ Snippets collection that can be an invaluable resource. It has snippets for HTML, CSS, htaccess, PHP JavaScript, jQuery and WordPress.
Even if you don't have a specific need it's well worth looking through for inspiration.
http://css-tricks.com/snippets/
I came across this post by Jean-Baptiste Jung yesterday, with some CSS tools I wasn't aware of. Worth checking out.
http://www.catswhocode.com/blog/super-useful-tools-for-css-coding