Fury & Sassy
While fads come and go, one that I personally have stuck with since it's inception is Foundation by Zurb. Foundation is a front-end CSS/JS framework that makes it crazy easy to prototype and develop sites/apps. It's amazing to look back at what Foundation once was and what it has become - and for me, it's an integral part of my day to day workflow.
Foundation has not only helped me developer faster, but smarter. Sass, the CSS pre-processor behind Foundation has guided me in creating more structured base setups; taking advantage of the plethora of amazing features it offers, like mixins, @extend (ZOMG) and variables.
There's tons of ways to get started using Sass. If you're on a Mac (which everyone should be nowadays), Codekit gives you all the benefits of compilation, concatination and minification — without the need for installing system tools. Windows users can even join the fun with Scout. And for all the terminal junkies out there like me, GruntJS fits the bill quite nicely.
One thing to keep in mind when using frameworks like Foundation or Boostrap is that these are simply toolkits - not turnkey solutions. Just because you can, doesn't mean you always should - so be original while using these frameworks. In-fact, I probably use about 15-25% of Foundation, but it's those reusable parts that have made development that much more enjoyable.