LATEST SOLUTION (2022)

Solution 4 using Bootstrap 4 or 5

Bootstrap 4 and 5 use Flexbox by default, so there is no need for extra CSS.

Demo

<div class="container">
    <div class="row ">
        <div class="col-md-4" style="background-color: red">
          some content
        </div>
        <div class="col-md-4" style="background-color: yellow">
          catz
          <img width="100" height="100" src="https://placekitten.com/100/100/">
        </div>
        <div class="col-md-4" style="background-color: green">
          some more content
        </div>
    </div>
</div>

Solution 1 using negative margins (doesn’t break responsiveness)

Demo

.row{
    overflow: hidden; 
}

[class*="col-"]{
    margin-bottom: -99999px;
    padding-bottom: 99999px;
}

Solution 2 using table

Demo

.row {
    display: table;
}

[class*="col-"] {
    float: none;
    display: table-cell;
    vertical-align: top;
}

Solution 3 using flex added August 2015. Comments posted before this don’t apply to this solution.

Demo

.row {
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display:         flex;
  flex-wrap: wrap;
}
.row > [class*='col-'] {
  display: flex;
  flex-direction: column;
}