How to vertically align floating divs to the bottom?

This will do the trick:

#bars {
    display: table-cell;
    border: solid 1px black;
}
#bars > div {
    display: inline-block;
    vertical-align: bottom;
    width: 5px;
    background-color: #999;
    margin-left: 2px;
}
#bars > div:first-child {
    margin-left: 0;
}

It uses display: table-cell; on the parent div, which by default has vertical-align: baseline; applied. This changes the need for float: left; on the child divs and allows us to use display: inline-block;. This also removes the need for your CSS clear fix.

EDIT - Per @thirtydot's comments, adding vertical-align: bottom; to the child divs removes the gap at the bottom.

Therefore, I changed CSS above and jsFiddle. I kept the display: table-cell; so that the parent div wraps the child divs with 0 padding and looks nice and snazzy!

FLEXBOX! Flexbox is the most bestest.

Example: http://jsfiddle.net/rudiedirkx/7FGKN/

Flexbox makes this ridiculously simple (and not to forget correct):

#container {
  display: flex; /* or inline-flex */
  flex-flow: row nowrap; /* is default: columns along the main-axis (row) and no wrapping to next lines */
  align-items: flex-end; /* bottom */
}
#container > div {
  /* margin etc here, but nothing layoutish */
}

That's it Two lines of CSS: display: flex and align-items: flex-end.