It all depends on your habits.

If you're satisfied with pure CSS and Javascript, the only thing you really need is a build tool for concatenation, minification and gzip. And livereload. And probably normalize.css.

-> I use uglifyjs and uglifycss

-> I use tiny-lr for livereload (https://www.npmjs.org/package/tiny-lr)

-> I use a Makefile, because it's the most simple to modify and adapt. When I have several targets of the same type (ex: build.min.js and debug.min.js), I use a yaml file with a custom python script for concatenation. See gulp.js and a thousand others for alternatives, YMMV (my Makefile nowadays: https://gist.github.com/idlewan/11012492). Magical bit for running make every time a file changes:

  watch:
      @inotifywait -m --exclude ".*\.swp|.*~" -e moved_to,close_write -r . | \
               while read; do make -s; done

To get more from just CSS, you need a CSS precompiler.

-> I use Stylus with the nib library (https://github.com/visionmedia/nib). Alternatives: LESS, SASS, and some others. You might want to add some grid and 'responsive shortcut' mixins on top.

  bigger($minwidth)
    query = "screen and (min-width: " + $minwidth + ")"
    @media query
      {block}

  // usage:
  .my-div
    font-size 20px
    +bigger(520px)
      font-size 30px

For webapp needs, you need a template library (some people prefer using a fat framework that does everything for them, e.g. Angular).

-> I use Jade (https://github.com/visionmedia/jade), because it runs on the server for Python, Node.js and soon Nimrod, and on the client with precompiled templates. Alternatives: Mustache and a thousand others, YMMV.